From e1e43c32818bf5a017c8cb0841949e4278955d5a Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Mon, 26 Oct 2026 18:51:29 -0400
Subject: [PATCH 01/10] Scope page author stylesheet variants to routes in
 place

---
 .../src/ArtifactCompiler/ArtifactCompiler.php |  35 +++--
 .../src/Css/CssSelectorMatcher.php            |   6 +-
 .../src/Css/CssStylesheetTransformer.php      | 142 ++++++++++++++++--
 .../WordPressSitePlan/DocumentRootContext.php |   9 +-
 .../WordPressSitePlan/WordPressSitePlan.php   |  22 +++
 .../tests/contract/context-seeded-markers.php |  22 +--
 .../tests/contract/document-head-runtime.php  |  12 +-
 .../page-stylesheet-dead-richtext-markers.php |   2 +-
 .../shared-author-stylesheet-scaling.php      | 130 ++++++++++++++--
 .../artifact-author-stylesheet-projection.php |  30 ++--
 .../unit/document-root-font-ownership.php     |   6 +-
 ...dpress-compat-nav-anchor-replay-growth.php |   2 +-
 12 files changed, 331 insertions(+), 87 deletions(-)

diff --git a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
index 9c5c48249..8584036ea 100644
--- a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
+++ b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
@@ -356,7 +356,7 @@ private function finalizeArtifact(array $artifact, array $reduction): Transforme
             $pageMarkup[(string) ($ownedBy(array(array()), (string) $sourcePath)[0]['owner'] ?? '')] = $document['serialized_blocks'];
         }
         $authoredFiles = $normalized['files'];
-        $normalized['files'] = $this->applyAuthorStylesheetProjections($normalized['files'], $authorStylesheetProjections, $entryBlocks['author_stylesheet_projections'], (string) ($ownedBy(array(array()), $entryPath)[0]['owner'] ?? ''), $pageMarkup);
+        $normalized['files'] = $this->applyAuthorStylesheetProjections($normalized['files'], $authorStylesheetProjections, $entryBlocks['author_stylesheet_projections'], (string) ($ownedBy(array(array()), $entryPath)[0]['owner'] ?? ''), $pageMarkup, $allDiagnostics);
         foreach ($normalized['files'] as $document) {
             if ('html' !== ($document['kind'] ?? '') || !is_string($document['content'] ?? null) || !empty($document['metadata']['compilation']['included_component'])) continue;
             $documentFiles = $normalized['files'];
@@ -2493,7 +2493,7 @@ private function allocateStylesheetOccurrencePath(string $candidate, array &$res
      * @param string $primaryOwner The page that produced the primary projections, '' when they are site-wide.
      * @return array<int, array<string, mixed>>
      */
-    private function applyAuthorStylesheetProjections(array $files, array $projections, array $primaryProjections = array(), string $primaryOwner = '', array $pageMarkup = array()): array
+    private function applyAuthorStylesheetProjections(array $files, array $projections, array $primaryProjections = array(), string $primaryOwner = '', array $pageMarkup = array(), array &$projectionDiagnostics = array()): array
     {
         $attributeStateMarkers = array();
         foreach ($projections as $projection) {
@@ -2548,6 +2548,12 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             }
         }
         $transformer = new CssStylesheetTransformer();
+        $routeIdentities = array();
+        foreach ($files as $document) {
+            if ('html' !== ($document['kind'] ?? '')) continue;
+            $source = (string) $document['path'];
+            $routeIdentities[$this->fileOwnership($document)['id']] = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\WordPressSitePlan::identity('page', $source, $source);
+        }
         $reserved = array_fill_keys(array_column($files, 'path'), true);
         $output = array();
         foreach ( $files as $file ) {
@@ -2613,12 +2619,8 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
                 }
             }
             $sharedStylesheets = array_merge(array_map('strval', array_keys($sharedByPath[$path] ?? array())), $stylesheets);
-            // A page's projection repeats every rule it did not rewrite. Only the
-            // rules missing from the site-wide copy go to a stylesheet the page
-            // alone loads. Place it after the site-wide copy: the latter may
-            // contain a generic selector that still matches the lowered block
-            // (e.g. a grid's default two-column rule), while the page projection
-            // carries the source's later attribute-specific responsive winner.
+            // Shared-shell projections are part of every route's stylesheet.
+            // Align the full route projection, never a trailing cascade delta.
             $native = array();
             foreach (array_merge(array_keys($sharedByPath[$path] ?? array()), $authoritativeContent) as $content) {
                 $native += $nativeByContent[$path][$content] ?? array();
@@ -2628,14 +2630,26 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             foreach ($ownersByPath[$path] ?? array() as $content => $owners) {
                 foreach ($owners as $owner => $_) $pageProjections[$owner][] = (string) $content;
             }
+            foreach ($pageProjections as &$contents) {
+                $contents = array($transformer->concatenateWithoutRedundantRules(array_merge(array_map('strval', array_keys($sharedByPath[$path] ?? array())), $contents)));
+            }
+            unset($contents);
+            $sharedStylesheets = array($transformer->concatenateWithoutRedundantRules($sharedStylesheets));
             $folded = $transformer->foldPositiveMarkerArms($sharedStylesheets, $pageProjections, $pageMarkup);
             $sharedStylesheets = $folded['shared'];
+            $fallbackSharedStylesheets = $sharedStylesheets;
             foreach ($pages as $owner => $_) $pages[$owner] = $folded['pages'][$owner] ?? $pages[$owner];
+            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $folded['pages'], $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null);
+            $sharedStylesheets = array($routed['stylesheet']);
+            foreach ($routed['scoped'] as $owner) unset($pages[$owner]);
+            foreach ($routed['reasons'] as $owner => $reason) {
+                $projectionDiagnostics[] = $this->diagnostic('author_stylesheet_route_scope_fallback', 'warning', $path . ' (' . $owner . '): ' . $reason);
+            }
             $projectedFile = $this->projectedStylesheetFile($file, $path, implode("\n", array_merge($preambles, array( $transformer->concatenateWithoutRedundantRules($sharedStylesheets) ))));
             $projectedFile['metadata']['native_selectors'] = array_keys($native);
             $output[] = $projectedFile;
             foreach ( $pages as $owner => $pageStylesheets ) {
-                $delta = $transformer->rulesAbsentFrom($pageStylesheets, $sharedStylesheets, true);
+                $delta = $transformer->rulesAbsentFrom($pageStylesheets, $fallbackSharedStylesheets, true);
                 if ( '' === trim($delta) ) {
                     continue;
                 }
@@ -4162,7 +4176,8 @@ private function fullDocumentMetadata(string $html, string $sourcePath, array $f
         }
         $titles = HtmlTagScanner::scan($html, 'title');
         $title = isset($titles[0]) ? trim(html_entity_decode(strip_tags($titles[0]['content']), ENT_QUOTES | ENT_HTML5, 'UTF-8')) : $this->titleFromHtml($html, $sourcePath);
-        $metadata = array('source_context' => array('source_path' => $sourcePath, 'kind' => 'html'), ...\Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::metadataFromHtml($html), 'title' => $title, 'title_declaration' => array('order' => 0, 'placement' => 'head'), 'meta' => $meta, 'links' => $links, 'scripts' => $scripts);
+        $routeIdentity = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\WordPressSitePlan::identity('page', $sourcePath, $sourcePath);
+        $metadata = array('source_context' => array('source_path' => $sourcePath, 'kind' => 'html'), ...\Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::metadataFromHtml($html, $routeIdentity), 'title' => $title, 'title_declaration' => array('order' => 0, 'placement' => 'head'), 'meta' => $meta, 'links' => $links, 'scripts' => $scripts);
         if ( array() !== $projectedRootMarkers ) {
             $rootClasses = array_values(array_filter(preg_split('/\s+/', trim((string) ($metadata['root_attributes']['class'] ?? ''))) ?: array(), static fn(string $class): bool => '' !== $class));
             $metadata['root_attributes']['class'] = implode(' ', array_unique(array_merge($rootClasses, $projectedRootMarkers)));
diff --git a/php-transformer/src/Css/CssSelectorMatcher.php b/php-transformer/src/Css/CssSelectorMatcher.php
index 02f31be5f..2f60f2e1a 100644
--- a/php-transformer/src/Css/CssSelectorMatcher.php
+++ b/php-transformer/src/Css/CssSelectorMatcher.php
@@ -519,7 +519,7 @@ private static function parseCompound(string $source, int $sourceStart, bool $is
                         // A neutral native target can be a complete descendant
                         // path, or contain nested shims. Flattening it into one
                         // compound changes both its match and its specificity.
-                        if (($argument['supported'] ?? false) && (count($argument['compounds']) > 1 || !empty($leaf['not']) || !empty($leaf['any']) || !empty($leaf['first_child']) || !empty($leaf['last_child']) || null !== ($leaf['nth_child'] ?? null) || null !== ($leaf['nth_type'] ?? null))) {
+                        if (($argument['supported'] ?? false) && (count($argument['compounds']) > 1 || !empty($leaf['root']) || !empty($leaf['not']) || !empty($leaf['any']) || !empty($leaf['first_child']) || !empty($leaf['last_child']) || null !== ($leaf['nth_child'] ?? null) || null !== ($leaf['nth_type'] ?? null))) {
                             $group = self::parseSelectorListArgument($alternatives, 'where' === $lowerName);
                             if (null === $group) return null;
                             $compound['any'][] = $group;
@@ -582,7 +582,9 @@ private static function parseCompound(string $source, int $sourceStart, bool $is
                     if ( '' === $argument ) {
                         return null;
                     }
-                    $negated = self::parseUncached($argument);
+                    $alternatives = CssStylesheetTransformer::splitSelectorList($argument);
+                    if (null === $alternatives) return null;
+                    $negated = self::parseUncached(count($alternatives) > 1 ? ':is(' . $argument . ')' : $argument);
                     if ( ! ($negated['supported'] ?? false) || null !== ($negated['pseudo_state_suffix_span'] ?? null) ) {
                         return null;
                     }
diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 13d5cae2d..8e046c29a 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -281,14 +281,15 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
             }
         }
         foreach ($index as $key => $matches) {
-            if (1 !== count($matches)) continue;
+            if (1 !== count(array_unique(array_map(static fn (int $i): string => $sharedUnits[$i]['text'], $matches)))) continue;
             $i = $matches[0];
             $base = $describe($sharedUnits[$i]);
             $union = $base['arms'];
             $candidates = array();
             foreach ($pageIndex as $owner => $rules) {
-                if (1 !== count($rules[$key] ?? array())) continue;
-                $description = $describe($pageUnits[$owner][$rules[$key][0]]);
+                $pageMatches = $rules[$key] ?? array();
+                if (!$pageMatches || 1 !== count(array_unique(array_map(static fn (int $j): string => $pageUnits[$owner][$j]['text'], $pageMatches)))) continue;
+                $description = $describe($pageUnits[$owner][$pageMatches[0]]);
                 foreach (array_diff($description['arms'], $union) as $arm) {
                     $marker = $markerOf($arm);
                     if (null !== $marker) $candidates[$arm] = $marker;
@@ -299,20 +300,20 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
                 if (array_diff_key($pages, $markup)) $safe = false;
                 foreach ($carriers[$marker] ?? array() as $owner => $_) {
                     $rules = $pageIndex[$owner][$key] ?? array();
-                    if (1 !== count($rules) || !in_array($arm, $describe($pageUnits[$owner][$rules[0]])['arms'], true)) { $safe = false; break; }
+                    if (!$rules) $safe = false;
+                    foreach ($rules as $j) if (!in_array($arm, $describe($pageUnits[$owner][$j])['arms'], true)) $safe = false;
+                    if (!$safe) break;
                 }
                 if ($safe) $union[] = $arm;
             }
             if ($union === $base['arms']) continue;
-            $sharedUnits[$i]['text'] = implode(',', $union) . substr($sharedUnits[$i]['text'], $base['brace']);
+            foreach ($matches as $match) $sharedUnits[$match]['text'] = implode(',', $union) . substr($sharedUnits[$match]['text'], $base['brace']);
             foreach ($pageIndex as $owner => $rules) {
-                if (1 !== count($rules[$key] ?? array())) continue;
-                $j = $rules[$key][0];
-                $description = $describe($pageUnits[$owner][$j]);
-                // Leave the entire rule on the old delta path if any arm was
-                // not safe to fold; its later cascade dependencies still matter.
-                if (array() === array_diff($description['arms'], $union)) {
-                    $pageUnits[$owner][$j]['text'] = $sharedUnits[$i]['text'];
+                foreach ($rules[$key] ?? array() as $j) {
+                    $description = $describe($pageUnits[$owner][$j]);
+                    if (array() === array_diff($description['arms'], $union)) {
+                        $pageUnits[$owner][$j]['text'] = $sharedUnits[$i]['text'];
+                    }
                 }
             }
         }
@@ -321,6 +322,123 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
         return $result;
     }
 
+    /**
+     * Reconcile ordered page projections at the source cascade position.
+     * Unsupported pages are returned separately for the legacy copy path.
+     *
+     * @param list<string> $shared
+     * @param array<string,list<string>> $pages
+     * @param array<string,string> $routeIdentities Reconciliation identities by compilation owner.
+     * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>}
+     */
+    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null): array
+    {
+        $base = array();
+        foreach ($shared as $css) $this->collectRuleUnits($css, array(), $base);
+        $key = static function (array $unit): string {
+            $brace = strpos($unit['text'], '{');
+            return serialize(array($unit['context'], false === $brace || str_starts_with(ltrim($unit['text']), '@') ? trim($unit['text']) : substr($unit['text'], $brace)));
+        };
+        $positions = array();
+        foreach ($base as $i => $unit) $positions[$key($unit)][] = $i;
+        $changes = $insertions = $reasons = array();
+        $scoped = array();
+        foreach ($pages as $owner => $sheets) {
+            $units = array();
+            $reason = $unsafeReason;
+            foreach (array_merge($shared, $sheets) as $css) {
+                if (!$this->isWellFormedStylesheet($css)) $reason = 'malformed stylesheet';
+                if (preg_match('/@namespace\b/i', $css)) $reason = '@namespace';
+            }
+            if (null !== $reason) { $reasons[$owner] = $reason; continue; }
+            foreach ($sheets as $css) $this->collectRuleUnits($css, array(), $units);
+            $cursor = 0;
+            $localChanges = $localInsertions = array();
+            foreach ($units as $unit) {
+                $match = null;
+                foreach ($positions[$key($unit)] ?? array() as $position) {
+                    if ($position >= $cursor) { $match = $position; break; }
+                }
+                if (null === $match) {
+                    $localInsertions[$cursor][] = $unit;
+                    continue;
+                }
+                while ($cursor < $match) $localChanges[$cursor++] = null;
+                if (trim($unit['text']) !== trim($base[$match]['text'])) $localChanges[$match] = $unit;
+                $cursor = $match + 1;
+            }
+            while ($cursor < count($base)) $localChanges[$cursor++] = null;
+            // Validate every affected selector before committing any route edits.
+            foreach ($localChanges as $i => $variant) {
+                foreach (array_filter(array($base[$i], $variant)) as $unit) {
+                    $this->scopeRouteUnit($unit, array('be-route-probe'), true, $reason);
+                }
+            }
+            foreach ($localInsertions as $list) foreach ($list as $unit) {
+                $this->scopeRouteUnit($unit, array('be-route-probe'), true, $reason);
+            }
+            if (null !== $reason) { $reasons[$owner] = $reason; continue; }
+            $scoped[] = (string) $owner;
+            foreach ($localChanges as $i => $variant) $changes[$i][$owner] = $variant;
+            foreach ($localInsertions as $i => $list) $insertions[$i][$owner] = $list;
+        }
+        $output = array();
+        $routeClass = static fn ($owner): string => \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($routeIdentities[$owner] ?? (string) $owner);
+        $emitGroups = function (array $groups) use (&$output, $routeClass): void {
+            $texts = array();
+            foreach ($groups as $owner => $units) foreach ($units as $ordinal => $unit) {
+                $groupKey = serialize(array($ordinal, $unit));
+                $texts[$groupKey]['unit'] = $unit;
+                $texts[$groupKey]['routes'][] = $routeClass($owner);
+            }
+            foreach ($texts as $group) $output[] = $this->scopeRouteUnit($group['unit'], $group['routes'], true);
+        };
+        for ($i = 0; $i <= count($base); ++$i) {
+            $emitGroups($insertions[$i] ?? array());
+            if ($i === count($base)) break;
+            $variants = $changes[$i] ?? array();
+            $output[] = $variants ? $this->scopeRouteUnit($base[$i], array_map($routeClass, array_keys($variants)), false) : $base[$i];
+            $emitGroups(array_map(static fn ($unit): array => null === $unit ? array() : array($unit), $variants));
+        }
+        return array('stylesheet' => $this->serializeRuleUnits($output), 'scoped' => $scoped, 'reasons' => $reasons);
+    }
+
+    /** @param array{context:list<string>,text:string,dedupable:bool} $unit @param list<string> $routes */
+    private function scopeRouteUnit(array $unit, array $routes, bool $positive, ?string &$reason = null): array
+    {
+        $brace = strpos($unit['text'], '{');
+        if (false === $brace || str_starts_with(ltrim($unit['text']), '@') || !$unit['dedupable']) { $reason = 'non-selector rule cannot be route scoped'; return $unit; }
+        $arms = self::splitSelectorList(trim(substr($unit['text'], 0, $brace)));
+        if (null === $arms) { $reason = 'unparseable selector list'; return $unit; }
+        $scope = ':where(:root:' . ($positive ? 'is' : 'not') . '(' . implode(',', array_map(static fn (string $route): string => '.' . $route, $routes)) . '))';
+        $rootAlternatives = array();
+        foreach ($arms as &$arm) {
+            $arm = trim($arm);
+            $tokens = CssSelectorTokenizer::tokenize($arm);
+            if (!$tokens['supported'] || str_contains($arm, '&') || str_contains($arm, '|') || str_contains($arm, '/*')) { $reason = 'unparseable or relative selector'; return $unit; }
+            if (preg_match('/::|:(?:before|after|first-line|first-letter)\b/i', $arm)) { $reason = 'pseudo-element selector'; return $unit; }
+            if (!(CssSelectorMatcher::parse($arm)['supported'] ?? false)) { $reason = 'unparseable selector'; return $unit; }
+            $first = $tokens['compounds'][0];
+            if (preg_match('/^(?:html(?=[.#:\[]|$)|:root(?=[.#:\[]|$))/i', $first)) {
+                $end = $tokens['compound_spans'][0]['end'];
+                $arm = substr($arm, 0, $end) . $scope . substr($arm, $end);
+            } else {
+                // Root tests hidden in functional selectors can match html too.
+                if (preg_match('/(?<![\w-])(?:html|:root)\b/i', $first)) { $reason = 'functional root selector'; return $unit; }
+                // Class/universal/attribute arms may themselves match html.
+                // Retain that match as well as descendants without adding weight.
+                if (preg_match('/^[*.#\[:]/', $first)) {
+                    $end = $tokens['compound_spans'][0]['end'];
+                    $rootAlternatives[] = substr($arm, 0, $end) . $scope . substr($arm, $end);
+                }
+                $arm = $scope . ' ' . $arm;
+            }
+        }
+        unset($arm);
+        $unit['text'] = implode(',', array_merge($arms, $rootAlternatives)) . substr($unit['text'], $brace);
+        return $unit;
+    }
+
     /**
      * The cascade families a rule unit declares: a property, without vendor
      * prefix, reduced to the shorthand that also sets it, so a shorthand and
diff --git a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
index 5d9b793f0..26c193fc0 100644
--- a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
+++ b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
@@ -17,12 +17,19 @@ public static function fromHtml(string $html, string $tagName = 'html'): array
     }
 
     /** @return array{root_attributes:array<string,string>,body_attributes:array<string,string>} */
-    public static function metadataFromHtml(string $html): array
+    public static function metadataFromHtml(string $html, string $routeIdentity = ''): array
     {
         $attributes = DocumentRootAttributes::forDocument($html);
+        if ('' !== $routeIdentity) $attributes['html']['class'] = trim(($attributes['html']['class'] ?? '') . ' ' . self::routeClass($routeIdentity));
         return array('root_attributes' => $attributes['html'], 'body_attributes' => $attributes['body']);
     }
 
+    /** Stable document compilation identity, shared by CSS and root reconciliation. */
+    public static function routeClass(string $identity): string
+    {
+        return 'be-route-' . substr(hash('sha256', $identity), 0, 12);
+    }
+
     public static function assertValid(mixed $attributes): void
     {
         if (!is_array($attributes)) throw new InvalidArgumentException('Document root context must be an attribute map.');
diff --git a/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php b/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php
index 9ef531467..d92ace297 100644
--- a/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php
+++ b/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php
@@ -453,6 +453,28 @@ public function fromCompilerInput(array $data, WordPressSitePlanInput $input): a
         unset($template);
         $runtimeDeclarations = $this->canonicalEntityBindings($runtimeDeclarations, $runtimeEntityRecords, $references, $routeMap, $pages, $bindingTemplates, $paginationOwners);
         $pages = $this->pageHierarchy($pages, $routeMap);
+        // Compilation knows source identities; route reconciliation finalizes
+        // them here. Remap the static CSS and the existing root registry together.
+        $routeClasses = array();
+        foreach ($pages as &$page) {
+            if (!empty($page['synthetic'])) continue;
+            $sourceClass = DocumentRootContext::routeClass(self::identity('page', $page['source_path'], $page['source_path']));
+            $routeClass = DocumentRootContext::routeClass($page['reconciliation_identity']);
+            $routeClasses[$sourceClass] = $routeClass;
+            $root = $page['document_metadata']['root_attributes'] ?? array();
+            $root['class'] = trim(str_replace($sourceClass, $routeClass, $root['class'] ?? ''));
+            if (!in_array($routeClass, explode(' ', $root['class']), true)) $root['class'] = trim($root['class'] . ' ' . $routeClass);
+            $page['document_metadata']['root_attributes'] = $root;
+        }
+        unset($page);
+        foreach ($assets as &$asset) {
+            if ('css' !== ($asset['kind'] ?? '') || !is_string($asset['content'] ?? null)) continue;
+            $asset['content'] = strtr($asset['content'], $routeClasses);
+            $asset['bytes'] = strlen($asset['content']);
+            $asset['hash'] = hash('sha256', $asset['content']);
+            $asset['content_hash'] = self::contentHash($asset['content']);
+        }
+        unset($asset);
         $templateOwners = array();
         foreach ($pages as $page) if (empty($page['entrypoint'])) $templateOwners[self::routeTemplateKey($page)][] = $page['source_path'];
         // Colliding terminal slugs get a readable route-qualified template
diff --git a/php-transformer/tests/contract/context-seeded-markers.php b/php-transformer/tests/contract/context-seeded-markers.php
index a566c050c..efe9294d1 100644
--- a/php-transformer/tests/contract/context-seeded-markers.php
+++ b/php-transformer/tests/contract/context-seeded-markers.php
@@ -137,11 +137,7 @@
 $assert(isset($indexColors['Brand'], $indexColors['Copy'], $indexColors['Foot']) && ! in_array('#f00', $indexColors, true) && '#123' === ($indexColors['Copy'] ?? null), "another page's own type rule reaches neither the chrome nor the content of this page", (string) json_encode($indexColors));
 $assert('#f00' === ($aboutColors['Brand'] ?? null) && '#f00' === ($aboutColors['Foot'] ?? null), "the page's own type rule still reaches its own chrome", (string) json_encode($aboutColors));
 
-// 6. The site-wide copy is the projection most pages share; the pages that
-//    need the same extra slice share one asset, enqueued on each of them
-//    right after the site-wide copy. `main>.actions .btn` depends on where
-//    the actions sit in `main`, so its control arm stays per element and
-//    differs between the two layouts.
+// 6. Both layouts keep their context-dependent arms in one shared asset.
 $sliced = $plan(array(
     'index.html' => $document('<main><p id="lead">Lead</p>' . $chrome . '</main>'),
     'd.html' => $document('<main><p id="lead">Lead</p>' . $chrome . '</main>'),
@@ -158,18 +154,10 @@
         $slices[] = $asset;
     }
 }
-$assert('' !== $siteCopy && 1 === count($slices) && array( 'index.html', 'd.html' ) === array_column($slices[0]['scopes'], 'source_path'), 'the two pages that need the same slice share one published asset', (string) json_encode(array_map(static fn (array $slice): array => array_column($slice['scopes'], 'source_path'), $slices)));
-foreach ( array( 'index.html', 'd.html', 'a.html', 'b.html', 'c.html' ) as $source ) {
-    $own = array_values(array_filter($slices, static fn (array $slice): bool => in_array($source, array_column($slice['scopes'], 'source_path'), true)));
-    // A stylesheet split for visitors has an editor copy; each context loads one.
-    foreach ( array( 'frontend', 'editor' ) as $context ) {
-        $rows = array_values(array_filter(WordPressSitePlan::documentStylesheets($sliced, $source), static fn (array $row): bool => in_array($row['stylesheet_target'], array( 'both', $context ), true)));
-        $sequence = array_column($rows, 'target_path');
-        $at = array_search($siteCopy, $sequence, true);
-        $slice = array() === $own ? null : $own[0]['target_path'];
-        $sliceIn = $slice;
-        $assert(false !== $at && count($own) <= 1 && (null === $sliceIn || ($sequence[$at + 1] ?? null) === $sliceIn), "{$source} enqueues its slice right after the site-wide copy ({$context})", (string) json_encode($sequence));
-    }
+$assert('' !== $siteCopy && array() === $slices, 'different layouts publish one stylesheet without page slices');
+foreach (array('index.html', 'd.html', 'a.html', 'b.html', 'c.html') as $source) {
+    $sequence = array_column(WordPressSitePlan::documentStylesheets($sliced, $source), 'target_path');
+    $assert(in_array($siteCopy, $sequence, true), $source . ' loads the shared route-scoped stylesheet');
 }
 
 if ( 0 < $failures ) {
diff --git a/php-transformer/tests/contract/document-head-runtime.php b/php-transformer/tests/contract/document-head-runtime.php
index 9a00f1af6..d933aca18 100644
--- a/php-transformer/tests/contract/document-head-runtime.php
+++ b/php-transformer/tests/contract/document-head-runtime.php
@@ -247,7 +247,7 @@ function _block_template_viewport_meta_tag(): void { print '<meta name="viewport
 foreach ($afterAuthor as $row) $assert(1 === substr_count($orderedEmitted, 'data-printed-handle="' . $row['handle'] . '"') && strpos($orderedEmitted, 'data-printed-handle="' . $row['handle'] . '"') > strpos($orderedEmitted, ' id="author"'), 'After-author support ' . $row['handle'] . ' prints once, after the replayed head stylesheet.');
 $assert(array() === array_filter($enqueuedStyles, static fn(string $src): bool => str_contains($src, 'inline')), 'The replayed head stylesheet has no remaining WordPress enqueue.');
 
-// #3508: projected companions must retain their positions between replayed links.
+// Route variants retain source order between replayed stylesheet links.
 $companions = array('entrypoint' => 'index.html', 'files' => array(
     'index.html' => '<!doctype html><html><head><link id="a" rel="stylesheet" href="a.css"><meta name="fixture" content="order"><link id="b" rel="stylesheet" href="b.css"></head><body><main><p class="sample">Sample</p></main></body></html>',
     'other.html' => '<!doctype html><html><head><link id="a" rel="stylesheet" href="a.css"><link id="b" rel="stylesheet" href="b.css"></head><body><main><p><span class="sample">Sample</span></p></main></body></html>',
@@ -260,8 +260,8 @@ function _block_template_viewport_meta_tag(): void { print '<meta name="viewport
 $plan['pages'] = array_reverse($plan['pages']);
 $sequence = WordPressSitePlan::documentStylesheets($plan, 'other.html');
 $authorSequence = array_values(array_filter($sequence, static fn(array $row): bool => 1 === preg_match('~/[ab](?:\\.page-[^.]+)?\\.css$~', $row['target_path'])));
-$assert(4 === count($authorSequence), 'The two-source fixture produces two source stylesheets and two projected companions.');
-$assert(1 === preg_match('~/a\\.css$~', $authorSequence[0]['target_path']) && str_contains($authorSequence[1]['target_path'], '/a.page-') && 1 === preg_match('~/b\\.css$~', $authorSequence[2]['target_path']) && str_contains($authorSequence[3]['target_path'], '/b.page-'), 'The enqueue order is a.css, a.page.css, b.css, b.page.css.');
+$assert(2 === count($authorSequence), 'The two-source fixture keeps route projections in the two source stylesheets.');
+$assert(1 === preg_match('~/a\\.css$~', $authorSequence[0]['target_path']) && 1 === preg_match('~/b\\.css$~', $authorSequence[1]['target_path']), 'The enqueue order remains a.css, b.css without projected companions.');
 $hooks = array(); $enqueuedStyles = array();
 add_action('wp_head', 'wp_print_styles', 8);
 eval(substr(array_column($plan['writes'], null, 'target_path')['functions.php']['payload']['data'], 5));
@@ -274,13 +274,13 @@ function _block_template_viewport_meta_tag(): void { print '<meta name="viewport
 $companionHead = ob_get_clean();
 $positions = array();
 foreach ($authorSequence as $index => $stylesheet) {
-    $needle = 0 === $index % 2 ? 'href="https://example.test/theme/' . $stylesheet['target_path'] . '"' : 'data-printed-handle="' . $stylesheet['handle'] . '"';
-    $assert(1 === substr_count($companionHead, $needle), 'Each source stylesheet and companion prints exactly once.');
+    $needle = 'href="https://example.test/theme/' . $stylesheet['target_path'] . '"';
+    $assert(1 === substr_count($companionHead, $needle), 'Each source stylesheet prints exactly once.');
     $positions[] = strpos($companionHead, $needle);
 }
 $sortedPositions = $positions;
 sort($sortedPositions);
-$assert($positions === $sortedPositions, 'Rendered stylesheet order is a.css, a.page.css, b.css, b.page.css.');
+$assert($positions === $sortedPositions, 'Rendered stylesheet order is a.css, b.css.');
 
 $repeated = $artifact;
 $repeated['files']['index.html'] = str_replace('</body>', '<script src="check.js"></script></body>', $repeated['files']['index.html']);
diff --git a/php-transformer/tests/contract/page-stylesheet-dead-richtext-markers.php b/php-transformer/tests/contract/page-stylesheet-dead-richtext-markers.php
index fb8f30c05..94cf81f1c 100644
--- a/php-transformer/tests/contract/page-stylesheet-dead-richtext-markers.php
+++ b/php-transformer/tests/contract/page-stylesheet-dead-richtext-markers.php
@@ -60,7 +60,7 @@
 
 // The reset still reaches every element, and the authored rules are unchanged.
 $all = implode("\n", array_map(static fn (array $asset): string => (string) $asset['content'], $stylesheets));
-$assert(str_contains($all, '*:not(:where(.blocks-engine-role-richtext)):not(.be-c-0),:where(.blocks-engine-role-richtext.blocks-engine-subject):not(.be-c-0){margin:0}'), 'The reset excludes the hidden label through the richtext role class and reaches it through the subject arm.');
+$assert(str_contains($all, '*:not(:where(.blocks-engine-role-richtext)):not(.be-c-0)') && str_contains($all, ':where(.blocks-engine-role-richtext.blocks-engine-subject):not(.be-c-0){margin:0}'), 'The route-scoped reset excludes the hidden label through the richtext role class and reaches it through the subject arm.');
 $assert(str_contains($all, '.cta{border:1px solid}'), 'Authored chrome rules are unchanged.');
 foreach (array('padding:2em', 'padding:1em') as $body) {
     $assert(str_contains($all, $body), 'The route rule `' . $body . '` is kept.');
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index d2e3e2159..3bd77c094 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -5,9 +5,8 @@
  * Generated theme CSS must not grow with pages x elements (#2755).
  *
  * N pages sharing one layout and one stylesheet compile that stylesheet to one
- * site-wide asset plus, at most, one slice shared by every page that needs the
- * same extra rules. Each page still resolves the same styles under its own
- * enqueue sequence.
+ * site-wide asset with route-scoped variants in source order. Each page still
+ * resolves the same styles under its own enqueue sequence.
  */
 
 require_once __DIR__ . '/../../vendor/autoload.php';
@@ -65,9 +64,7 @@
 // Ten pages that share a layout must not even double what two pages need.
 $assert($large['css_bytes'] < 2 * $small['css_bytes'], 'ten pages sharing a stylesheet need less than twice the generated CSS of two pages', $small['css_bytes'] . ' vs ' . $large['css_bytes']);
 
-// The shared stylesheet compiles to one site-wide asset, the projection most
-// pages share, whose bytes do not depend on how many pages consume it, plus at
-// most one slice for the page that differs.
+// Shared author CSS stays in one site-wide asset, including route differences.
 $stylesheetAssets = static function (array $plan): array {
     $site = array();
     $slices = array();
@@ -86,10 +83,8 @@
 [ $mediumSite, $mediumSlices ] = $stylesheetAssets($medium['plan']);
 [ $largeSite, $largeSlices ] = $stylesheetAssets($large['plan']);
 $assert(1 === count($mediumSite) && 1 === count($largeSite), 'the shared stylesheet compiles to one site-wide asset', (string) json_encode(array( count($mediumSite), count($largeSite) )));
-$assert(($mediumSite[0]['content'] ?? null) === ($largeSite[0]['content'] ?? ''), 'the site-wide asset is byte-identical however many pages consume it');
-$assert(count($largeSlices) <= 1 && count($largeSlices) === count($mediumSlices) && ($largeSlices[0]['content'] ?? null) === ($mediumSlices[0]['content'] ?? null), 'the page slice does not grow with the page count either', (string) count($largeSlices));
-$sliceScopes = array_map(static fn (array $scope): string => (string) ($scope['source_path'] ?? ''), $largeSlices[0]['scopes'] ?? array());
-$assert(array() === $largeSlices || array( 'a.html' ) === $sliceScopes, 'only the page whose own element a rule targets (#note-0) loads a slice', (string) json_encode($sliceScopes));
+$assert(!str_contains(implode('', array_column($large['plan']['assets'], 'source_path')), '.page-'), 'route variants never emit page-copy assets');
+$assert(array() === $largeSlices && array() === $mediumSlices, 'neither site needs page slices');
 
 // The control, the list and the shared chrome carry the same per-element
 // markers on every page; the per-page body class does not leak into them.
@@ -110,6 +105,7 @@
     }
     $document = new DOMDocument();
     @$document->loadHTML(StaticStyleParityRunner::candidateHtmlFromSerializedBlocks((string) ($planPage['canonical_block_markup'] ?? '')));
+    foreach ($planPage['document_metadata']['root_attributes'] ?? array() as $name => $value) $document->documentElement->setAttribute($name, $value);
     $cascade = new StaticCssCascade($document, $css);
     $probe = array();
     $button = $document->getElementsByTagName('a')->item(2) ?? $document->getElementsByTagName('button')->item(0);
@@ -186,7 +182,7 @@
 $twoAssets = $authoredAssets($variedTwo);
 $tenAssets = $authoredAssets($variedTen);
 $assert(1 === count($twoAssets) && 1 === count($tenAssets), 'the stylesheet compiles to one asset however different the pages are', (string) json_encode(array( array_keys($twoAssets), array_keys($tenAssets) )));
-$assert($twoAssets === $tenAssets, 'its bytes do not depend on the page count or on what the pages hold', strlen(implode('', $twoAssets)) . ' vs ' . strlen(implode('', $tenAssets)));
+$assert(strlen(implode('', $tenAssets)) < 2 * strlen(implode('', $twoAssets)), 'route lists grow without copying the stylesheet', strlen(implode('', $twoAssets)) . ' vs ' . strlen(implode('', $tenAssets)));
 $assert(! preg_match('/blocks-engine-(?:control|semantic)-[a-f0-9]{12}-\d+/', implode('', $tenAssets)), 'no arm of the shared stylesheet names a per-element marker', (string) (preg_match('/[^{}]*blocks-engine-(?:control|semantic)-[a-f0-9]{12}-\d+[^{}]*/', implode('', $tenAssets), $named) ? $named[0] : ''));
 
 $variedAssetsByTarget = array_column($variedTen['assets'] ?? array(), null, 'target_path');
@@ -198,6 +194,7 @@
     }
     $document = new DOMDocument();
     @$document->loadHTML('<?xml encoding="UTF-8"><html><body>' . StaticStyleParityRunner::candidateHtmlFromSerializedBlocks((string) ($planPage['canonical_block_markup'] ?? '')) . '</body></html>');
+    foreach ($planPage['document_metadata']['root_attributes'] ?? array() as $name => $value) $document->documentElement->setAttribute($name, $value);
     $cascade = new StaticCssCascade($document, $sheet);
     $probe = array();
     foreach ( $document->getElementsByTagName('*') as $element ) {
@@ -267,6 +264,7 @@
     $assets = array_column($contextTen['assets'], null, 'target_path');
     $css = '';
     foreach (WordPressSitePlan::documentStylesheets($contextTen, $planPage['source_path']) as $sheet) $css .= "\n" . ($assets[$sheet['target_path']]['content'] ?? '');
+    foreach ($planPage['document_metadata']['root_attributes'] ?? array() as $name => $value) $document->documentElement->setAttribute($name, $value);
     $cascade = new StaticCssCascade($document, $css);
     foreach ($document->getElementsByTagName('*') as $element) {
         if (!$element instanceof DOMElement || !str_contains($element->getAttribute('class'), 'wp-block-buttons')) continue;
@@ -302,14 +300,118 @@
 }
 
 // A reused identity is not proof of a shared match: both pages carry the
-// marker, but only one projected the arm. Keep that page's original delta.
+// marker, but only one projected the arm. Keep it out of the marker fold.
 $marker = 'blocks-engine-control-0123456789ab-0';
 $base = '.btn-group>.btn{z-index:1}.tail{z-index:2}';
 $extra = '.btn-group>.btn,:where(.' . $marker . ')>.wp-block-button__link{z-index:1}.tail{z-index:2}';
 $folded = $transformer->foldPositiveMarkerArms(array($base), array('a' => array($extra), 'b' => array($base)), array('a' => '<div class="' . $marker . '"></div>', 'b' => '<div class="' . $marker . '"></div>'));
 $assert(!str_contains(implode('', $folded['shared']), $marker), 'a marker carried by a non-projecting page is not folded');
-$delta = $transformer->rulesAbsentFrom($folded['pages']['a'], $folded['shared'], true);
-$assert(str_contains($delta, $marker) && str_contains($delta, '.tail{z-index:2}'), 'unsafe arm stays in its cascade-preserving page delta');
+$routed = $transformer->scopeRouteVariants($folded['shared'], $folded['pages']);
+$assert(str_contains($routed['stylesheet'], $marker) && 2 === count($routed['scoped']), 'an arm unsafe for marker folding can still be route scoped in place');
+
+// Real constant-arm variants, a dropped control override, and page-only
+// image bridge rules. Identical span routes must share one positive rule.
+$routeCss = 'html .sample{background-color:#123456}:root .sample{border-top-width:3px}'
+    . '@media(min-width:1px){button{padding:7px}.sample{color:red}.only{margin:9px}img{width:41px}}';
+for ($i = 0; $i < 100; ++$i) $routeCss .= '.cascade-tail-' . $i . '{color:#123456;padding:2px;border-top-width:1px;background-color:#fff}';
+$routeBodies = array(
+    '<button class="sample only">Probe</button>',
+    '<span class="sample">Probe</span>',
+    '<span class="sample">Probe</span>',
+    '<button class="sample only">Probe</button>',
+    '<button class="sample only">Probe</button><img src="photo.png" alt="Photo">',
+);
+$routeFiles = array();
+foreach ($routeBodies as $i => $body) $routeFiles[] = array('path' => chr(97 + $i) . '.html', 'kind' => 'html', 'content' => '<!doctype html><html><head><title>Routes</title><link rel="stylesheet" href="base.css"></head><body><main>' . $body . '</main></body></html>');
+$routeSheet = array('path' => 'base.css', 'kind' => 'css', 'content' => $routeCss);
+$routeResult = (new ArtifactCompiler())->compile(array('entrypoint' => 'a.html', 'files' => array_merge($routeFiles, array($routeSheet))))->toArray();
+$routePlan = $routeResult['source_reports']['wordpress_site_plan'];
+$routeAssets = $authoredAssets($routePlan);
+$routeText = implode('', $routeAssets);
+$assert(1 === count($routeAssets) && !str_contains(implode('', array_column($routePlan['assets'], 'source_path')), '.page-'), 'constant-arm variants, dropped rules and page-only rules need no copies');
+$classes = array();
+foreach ($routePlan['pages'] as $row) {
+    $class = Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($row['reconciliation_identity']);
+    $classes[$row['source_path']] = $class;
+    $assert(in_array($class, explode(' ', $row['document_metadata']['root_attributes']['class'] ?? ''), true), 'document root carries its reconciliation route class: ' . $row['source_path']);
+    $functions = array_column($routePlan['writes'], null, 'target_path')['functions.php']['payload']['data'];
+    $assert(str_contains($functions, $class) && str_contains($functions, 'blocksEngineDocumentContext') && str_contains($functions, 'language_attributes'), 'the same root registry reaches front end and editor: ' . $row['source_path']);
+}
+$group = ':where(:root:is(.' . $classes['b.html'] . ',.' . $classes['c.html'] . '))';
+$assert(str_contains($routeText, $group . ' .sample'), 'byte-identical constant-arm variants group both span routes');
+$assert(str_contains($routeText, 'html:where(:root:') && str_contains($routeText, ':root:where(:root:'), 'html and root scopes merge into the leftmost compound');
+$droppedOverride = false;
+$conditionalImage = false;
+$transformer->visitStyleRules($routeText, static function (string $prelude, string $body, array $context) use (&$droppedOverride, &$conditionalImage, $classes): void {
+    if (str_contains($body, 'padding:7px!important')) {
+        $droppedOverride = in_array('@media(min-width:1px)', $context, true) && str_contains($prelude, ':root:not(.' . $classes['b.html'] . ',.' . $classes['c.html'] . ')');
+    }
+    if (str_contains($prelude, '.wp-block-image') && str_contains($prelude, ':root:is(.' . $classes['e.html'] . ')')) $conditionalImage = in_array('@media(min-width:1px)', $context, true);
+});
+$assert($droppedOverride, 'the dropped override excludes both span routes at its original conditional position');
+$assert($conditionalImage, 'page-only image rules retain their at-rule context');
+$assert(str_contains($routeText, ':where(:root:is(.' . $classes['e.html'] . '))'), 'page-only image bridge has a positive route scope');
+
+// Each route is compared with its independent public compilation, including
+// editor wrapper selectors. Apply exactly the root attributes the theme uses.
+$probeStyles = static function (array $plan, array $row, bool $editor): array {
+    $assets = array_column($plan['assets'], null, 'target_path');
+    $css = '';
+    foreach (WordPressSitePlan::documentStylesheets($plan, $row['source_path']) as $sheet) $css .= "\n" . ($assets[$sheet['target_path']]['content'] ?? '');
+    $markup = StaticStyleParityRunner::candidateHtmlFromSerializedBlocks($row['canonical_block_markup']);
+    $dom = new DOMDocument();
+    @$dom->loadHTML('<html><body>' . ($editor ? '<div class="editor-styles-wrapper">' : '') . $markup . ($editor ? '</div>' : '') . '</body></html>');
+    foreach ($row['document_metadata']['root_attributes'] ?? array() as $name => $value) $dom->documentElement->setAttribute($name, $value);
+    $cascade = new StaticCssCascade($dom, $css);
+    $styles = array();
+    foreach ($dom->getElementsByTagName('*') as $element) {
+        if (!$element instanceof DOMElement) continue;
+        if ('img' === $element->tagName) {
+            $styles[] = $cascade->resolve($element, array('width', 'max-width', 'display'), array());
+        } elseif ('Probe' === trim($element->textContent) && in_array($element->tagName, array('p', 'span', 'a', 'button'), true)) {
+            $styles[] = $cascade->resolve($element, array('color', 'background-color', 'padding-top', 'border-top-width'), array());
+        }
+    }
+    return $styles;
+};
+foreach ($routePlan['pages'] as $row) {
+    $file = array_column($routeFiles, null, 'path')[$row['source_path']];
+    $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $file['path'], 'files' => array($file, $routeSheet)))->toArray()['source_reports']['wordpress_site_plan'];
+    foreach (array(false, true) as $editor) {
+        $actual = $probeStyles($routePlan, $row, $editor);
+        $expected = $probeStyles($alone, $alone['pages'][0], $editor);
+        $assert(array() !== $actual && $expected === $actual, 'route computed-style parity ' . $row['source_path'] . ($editor ? ' editor' : ' front end'), json_encode(array($expected, $actual)));
+    }
+}
+echo 'Route fixture author CSS: ' . strlen($routeText) . ' bytes in ' . count($routeAssets) . " assets\n";
+
+// @namespace requires per-sheet fallback, without preventing another sheet
+// on the same route from using in-place variants.
+$unsafeSheet = array('path' => 'unsafe.css', 'kind' => 'css', 'content' => '@namespace svg url("http://www.w3.org/2000/svg");' . $routeCss);
+$fallbackFiles = $routeFiles;
+foreach ($fallbackFiles as &$file) $file['content'] = str_replace('</head>', '<link rel="stylesheet" href="unsafe.css"></head>', $file['content']);
+unset($file);
+$fallback = (new ArtifactCompiler())->compile(array('entrypoint' => 'a.html', 'files' => array_merge($fallbackFiles, array($routeSheet, $unsafeSheet))))->toArray();
+$findings = array_filter($fallback['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '') && str_contains($row['message'], '@namespace') && str_contains($row['message'], 'unsafe.css'));
+$assert(array() !== $findings, 'unsafe stylesheet emits a page-attributed reason diagnostic');
+$fallbackAssets = $fallback['source_reports']['wordpress_site_plan']['assets'];
+$copies = array_filter($fallbackAssets, static fn (array $asset): bool => str_contains($asset['source_path'], '.page-'));
+$assert(array() !== $copies && array() === array_filter($copies, static fn (array $asset): bool => str_starts_with($asset['source_path'], 'base.page-')), 'fallback is restricted to the unsafe sheet');
+foreach (array(
+    array('.x::before{color:red}', '.y::before{color:red}', 'pseudo-element'),
+    array('.x{color:red}', '& .x{color:red}', 'relative'),
+    array('.x{color:red}', '.x{color:red', 'malformed'),
+) as [$base, $variant, $reason]) {
+    $routed = $transformer->scopeRouteVariants(array($base), array('unsafe' => array($variant)));
+    $assert(array() === $routed['scoped'] && str_contains($routed['reasons']['unsafe'] ?? '', $reason), 'unsafe route falls back: ' . $reason);
+}
+
+// Identical shared units are one fold match, rewritten at every occurrence.
+$marker = 'blocks-engine-control-0123456789ab-0';
+$unit = '.group>.btn{z-index:1}';
+$variant = '.group>.btn,:where(.' . $marker . '){z-index:1}';
+$duplicates = $transformer->foldPositiveMarkerArms(array($unit . '.between{z-index:2}' . $unit), array('a' => array($variant . '.between{z-index:2}' . $variant)), array('a' => '<p class="' . $marker . '">Probe</p>'));
+$assert(2 === substr_count(implode('', $duplicates['shared']), $marker), 'marker folding rewrites every byte-identical shared unit without moving it');
 
 if ( 0 < $failures ) {
     fwrite(STDERR, "shared-author-stylesheet scaling FAILED: {$passes} passed, {$failures} failed\n");
diff --git a/php-transformer/tests/unit/artifact-author-stylesheet-projection.php b/php-transformer/tests/unit/artifact-author-stylesheet-projection.php
index f161d9dd5..f98b1a54e 100644
--- a/php-transformer/tests/unit/artifact-author-stylesheet-projection.php
+++ b/php-transformer/tests/unit/artifact-author-stylesheet-projection.php
@@ -604,27 +604,15 @@
 $gridPageCss = implode('', array_column($gridPageAssets, 'content'));
 $gridBaseCss = false !== $gridBaseIndex ? (string) ($gridAssets[$gridBaseIndex]['content'] ?? '') : '';
 $gridMarker = '';
-preg_match('/blocks-engine-attribute-[a-f0-9]+-\d+/', $gridPageCss, $gridMarkerMatch);
+preg_match('/blocks-engine-attribute-[a-f0-9]+-\d+/', $gridBaseCss, $gridMarkerMatch);
 $gridMarker = $gridMarkerMatch[0] ?? '';
 $gridPages = array_column($responsiveGrid['source_reports']['compiled_site']['pages'] ?? array(), null, 'source_path');
 $gridMarkup = (string) ($gridPages['people.html']['block_markup'] ?? '');
-$assert(false !== $gridBaseIndex && count($gridPageAssets) === 1 && array_search($gridPageAssets[0]['path'], array_column($gridAssets, 'path'), true) > $gridBaseIndex, 'the page-specific grid projection follows its shared stylesheet');
-$gridPlanPaths = array_column($responsiveGrid['source_reports']['wordpress_site_plan']['assets'] ?? array(), 'source_path');
-$gridPlanBase = array_search('cards.css', $gridPlanPaths, true);
-$gridPlanPage = array_search($gridPageAssets[0]['path'] ?? '', $gridPlanPaths, true);
-$assert(false !== $gridPlanBase && false !== $gridPlanPage && $gridPlanBase < $gridPlanPage, 'the WordPress site plan preserves the shared-then-page stylesheet cascade through theme enqueue');
-$gridSource = 'cards.css';
-$gridPage = 'cards.page-123456789abc.css';
-$orderedGridStyles = ( new ReflectionMethod(\Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\WordPressSitePlan::class, 'orderPageStylesheetProjections') )->invoke(null, array(
-    array('kind' => 'css', 'source_path' => $gridPage, 'content' => '.cards:not([data-count="1"]){grid-template-columns:repeat(4,1fr)}'),
-    array('kind' => 'css', 'source_path' => $gridPage . '.shared-chrome', 'content' => '.shared{color:#456}'),
-    array('kind' => 'css', 'source_path' => 'unrelated.css', 'content' => '.other{color:#789}'),
-    array('kind' => 'css', 'source_path' => $gridSource, 'content' => '.cards:not([data-count="1"]){grid-template-columns:repeat(2,1fr)}'),
-    array('kind' => 'css', 'source_path' => $gridSource . '.shared-chrome', 'content' => '.shared{color:#123}'),
-));
-$orderedGridSources = array_column($orderedGridStyles, 'source_path');
-$assert(array('unrelated.css', $gridSource, $gridSource . '.shared-chrome', $gridPage, $gridPage . '.shared-chrome') === $orderedGridSources, 'a linked shared stylesheet stays before its page-scoped winner after chrome factoring without moving unrelated CSS');
-$assert('' !== $gridMarker && str_contains($gridPageCss, 'grid-template-columns:repeat(4,1fr)') && str_contains($gridPageCss, '@media(max-width:600px)') && str_contains($gridBaseCss, 'grid-template-columns:repeat(2,1fr)') && ! str_contains($gridPageCss, 'grid-template-columns:repeat(2,1fr)'), 'page projection retains its own responsive grid winners without replaying an unrelated shared two-column fallback after them');
+$assert(false !== $gridBaseIndex && array() === $gridPageAssets, 'the responsive grid projection stays in its shared stylesheet');
+$gridPlan = $responsiveGrid['source_reports']['wordpress_site_plan'];
+$gridPlanPages = array_column($gridPlan['pages'], null, 'source_path');
+$assert(!str_contains(implode('', array_column($gridPlan['assets'], 'source_path')), 'cards.page-'), 'the generated theme needs no page-specific grid copy');
+$assert('' !== $gridMarker && str_contains($gridBaseCss, 'grid-template-columns:repeat(4,1fr)') && str_contains($gridBaseCss, '@media(max-width:600px)') && str_contains($gridBaseCss, 'grid-template-columns:repeat(2,1fr)'), 'shared grid projection retains both layouts and their responsive winners');
 $assert(str_contains(implode("\n", array_column($gridAssets, 'content')), 'grid-gap:100px 100px') && str_contains($gridMarkup, 'be-inline-geometry-'), 'the editable grid keeps its inline-authored legacy gap through the existing layout carrier');
 $assert(str_contains($gridMarkup, 'blocks-engine-css-owned-grid') && str_contains($gridMarkup, $gridMarker) && 'pass' === ( new Runtime() )->validateBlockSerialization($gridMarkup)['status'], 'the attribute-selected four-card grid remains a valid editable Group document');
 $gridDocument = new DOMDocument();
@@ -636,9 +624,11 @@
         break;
     }
 }
-$gridCascade = new StaticCssCascade($gridDocument, $gridBaseCss . $gridPageCss);
+foreach ($gridPages['people.html']['document_metadata']['root_attributes'] ?? array() as $name => $value) $gridDocument->documentElement->setAttribute($name, $value);
+$gridCascade = new StaticCssCascade($gridDocument, $gridBaseCss);
 $assert($gridElement instanceof DOMElement && 'repeat(4,1fr)' === ($gridCascade->resolve($gridElement, array( 'grid-template-columns' ), array())['grid-template-columns'] ?? ''), 'the projected four-column rule wins the shared two-column fallback in source order at the desktop reference');
-$gridCascade = new StaticCssCascade($gridDocument, implode("\n", array_column($orderedGridStyles, 'content')));
+foreach ($gridPlanPages['people.html']['document_metadata']['root_attributes'] ?? array() as $name => $value) $gridDocument->documentElement->setAttribute($name, $value);
+$gridCascade = new StaticCssCascade($gridDocument, implode("\n", array_column($gridPlan['assets'], 'content')));
 $assert('repeat(4,1fr)' === ($gridCascade->resolve($gridElement, array('grid-template-columns'), array())['grid-template-columns'] ?? ''), 'the final site-plan stylesheet order preserves the four-column browser winner');
 
 $pageSubsetArtifact = array(
diff --git a/php-transformer/tests/unit/document-root-font-ownership.php b/php-transformer/tests/unit/document-root-font-ownership.php
index 2b3b9125e..9b9af3634 100644
--- a/php-transformer/tests/unit/document-root-font-ownership.php
+++ b/php-transformer/tests/unit/document-root-font-ownership.php
@@ -18,12 +18,12 @@
 )))->toArray();
 $plan = (new WordPressSitePlan())->fromCompilerResult($result);
 $pages = array_column($plan['pages'], null, 'source_path');
-$assert('type-a' === ($pages['index.html']['document_metadata']['root_attributes']['class'] ?? null), 'entry root scope travels through site plan');
-$assert('type-b' === ($pages['other/index.html']['document_metadata']['root_attributes']['class'] ?? null), 'route-specific scope stays separate');
+$assert(in_array('type-a', explode(' ', $pages['index.html']['document_metadata']['root_attributes']['class'] ?? ''), true), 'entry root scope travels through site plan');
+$assert(in_array('type-b', explode(' ', $pages['other/index.html']['document_metadata']['root_attributes']['class'] ?? ''), true), 'route-specific scope stays separate');
 $assert('document' === ($pages['index.html']['document_metadata']['root_attributes']['id'] ?? null), 'root ID travels alongside classes');
 $bootstrap = '';
 foreach ($plan['writes'] as $write) if ('functions.php' === $write['target_path']) $bootstrap = $write['payload']['data'];
-$assert(str_contains($bootstrap, "'language_attributes'") && str_contains($bootstrap, 'WP_HTML_Tag_Processor') && str_contains($bootstrap, "'type-a'") && str_contains($bootstrap, "'type-b'"), 'generated runtime adopts route-owned root identity through native HTML API');
+$assert(str_contains($bootstrap, "'language_attributes'") && str_contains($bootstrap, 'WP_HTML_Tag_Processor') && str_contains($bootstrap, 'type-a be-route-') && str_contains($bootstrap, 'type-b be-route-'), 'generated runtime adopts route-owned root identity through native HTML API');
 $assert(str_contains($bootstrap, "'_blocks_engine_reconciliation_identity'") && str_contains($bootstrap, 'is_front_page()'), 'runtime selection uses destination identity with route fallback');
 $css = implode("\n", array_column(array_filter($plan['assets'], static fn(array $asset): bool => 'css' === $asset['kind']), 'content'));
 $assert(str_contains($css, '.type-a') && str_contains($css, '.type-b') && str_contains($css, '@media'), 'authored root selector and conditional cascade stay intact');
diff --git a/php-transformer/tests/unit/wordpress-compat-nav-anchor-replay-growth.php b/php-transformer/tests/unit/wordpress-compat-nav-anchor-replay-growth.php
index b2d0b6684..2ae240ebe 100644
--- a/php-transformer/tests/unit/wordpress-compat-nav-anchor-replay-growth.php
+++ b/php-transformer/tests/unit/wordpress-compat-nav-anchor-replay-growth.php
@@ -64,7 +64,7 @@
         foreach ($result['assets'] as $asset) {
             if (isset($asset['stylesheet_parent'])) $parents[$asset['stylesheet_parent']] = true;
         }
-        $assert(isset($parents['site.css'], $parents['menu.css']), 'Multi-page growth fixture produces deltas for two different parent sheets.');
+        $assert(!isset($parents['site.css'], $parents['menu.css']), 'Both shared parent sheets keep their route variants in place.');
     }
     $plan = $result['source_reports']['wordpress_site_plan'];
     $sharedReplay = array_values(array_filter($plan['assets'], static fn (array $asset): bool => 'wordpress-compat' === ($asset['source'] ?? '')));

From 2591c905180f28049f058079b18bf30d7840984e Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Mon, 26 Oct 2026 19:24:03 -0400
Subject: [PATCH 02/10] Route-scope comment-led, pseudo-class, pseudo-element
 and at-rule variants

---
 .../src/Css/CssStylesheetTransformer.php      | 238 +++++++++++++++---
 .../WordPressSitePlan/DocumentRootContext.php |   2 +-
 .../WordPressSitePlan/WordPressSitePlan.php   |  22 --
 .../shared-author-stylesheet-scaling.php      |  55 +++-
 4 files changed, 259 insertions(+), 58 deletions(-)

diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 8e046c29a..19b287313 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -326,22 +326,37 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
      * Reconcile ordered page projections at the source cascade position.
      * Unsupported pages are returned separately for the legacy copy path.
      *
+     * Definition at-rules (`@keyframes`, `@font-face`, …) are inert unless a
+     * declaration references them, so the ones only some routes keep or add
+     * stay site-wide, once each. Two routes that define the same name with
+     * different bodies would see each other's definition, so they fall back.
+     *
      * @param list<string> $shared
      * @param array<string,list<string>> $pages
-     * @param array<string,string> $routeIdentities Reconciliation identities by compilation owner.
+     * @param array<string,string> $routeIdentities Route identities by compilation owner.
      * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>}
      */
     public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null): array
     {
         $base = array();
         foreach ($shared as $css) $this->collectRuleUnits($css, array(), $base);
-        $key = static function (array $unit): string {
-            $brace = strpos($unit['text'], '{');
-            return serialize(array($unit['context'], false === $brace || str_starts_with(ltrim($unit['text']), '@') ? trim($unit['text']) : substr($unit['text'], $brace)));
+        $normal = static fn (array $unit): string => trim(CssSyntaxScanner::withoutComments($unit['text']));
+        $key = function (array $unit) use ($normal): string {
+            $rule = self::splitLeadingComments($unit['text'])[1];
+            $brace = $this->nextRuleBoundary($rule, 0);
+            $selector = null !== $brace && '{' === $rule[$brace] && !$this->isAtRule($rule);
+            return serialize(array($unit['context'], $selector ? substr($rule, $brace) : $normal($unit)));
         };
-        $positions = array();
-        foreach ($base as $i => $unit) $positions[$key($unit)][] = $i;
-        $changes = $insertions = $reasons = array();
+        $value = static fn (array $unit): string => serialize(array($unit['context'], $normal($unit)));
+        $positions = $baseDefinitions = $defined = $baseValues = array();
+        foreach ($base as $i => $unit) {
+            $positions[$key($unit)][] = $i;
+            $baseDefinitions[$i] = $this->routeIndependentDefinition($unit);
+            if (null === $baseDefinitions[$i]) continue;
+            $defined[$baseDefinitions[$i]][$value($unit)] = true;
+            $baseValues[$value($unit)] = true;
+        }
+        $changes = $insertions = $siteInsertions = $reasons = array();
         $scoped = array();
         foreach ($pages as $owner => $sheets) {
             $units = array();
@@ -353,21 +368,39 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             if (null !== $reason) { $reasons[$owner] = $reason; continue; }
             foreach ($sheets as $css) $this->collectRuleUnits($css, array(), $units);
             $cursor = 0;
-            $localChanges = $localInsertions = array();
+            $localChanges = $localInsertions = $localSite = array();
             foreach ($units as $unit) {
                 $match = null;
                 foreach ($positions[$key($unit)] ?? array() as $position) {
                     if ($position >= $cursor) { $match = $position; break; }
                 }
                 if (null === $match) {
-                    $localInsertions[$cursor][] = $unit;
+                    if (null !== $this->routeIndependentDefinition($unit)) $localSite[$cursor][] = $unit;
+                    else $localInsertions[$cursor][] = $unit;
                     continue;
                 }
                 while ($cursor < $match) $localChanges[$cursor++] = null;
-                if (trim($unit['text']) !== trim($base[$match]['text'])) $localChanges[$match] = $unit;
+                if ($normal($unit) !== $normal($base[$match])) $localChanges[$match] = $unit;
                 $cursor = $match + 1;
             }
             while ($cursor < count($base)) $localChanges[$cursor++] = null;
+            // A definition the route drops or adds stays site-wide, unless the
+            // route would then see a different body under the same name.
+            $pageDefined = $defined;
+            $touched = array();
+            foreach ($localChanges as $i => $variant) {
+                if (null === $baseDefinitions[$i]) continue;
+                $touched[$baseDefinitions[$i]] = true;
+                unset($localChanges[$i]);
+            }
+            foreach ($localSite as $list) foreach ($list as $unit) {
+                $name = (string) $this->routeIndependentDefinition($unit);
+                $touched[$name] = true;
+                $pageDefined[$name][$value($unit)] = true;
+            }
+            foreach (array_keys($touched) as $name) {
+                if (count($pageDefined[$name] ?? array()) > 1) { $reason = 'conflicting ' . $name; break; }
+            }
             // Validate every affected selector before committing any route edits.
             foreach ($localChanges as $i => $variant) {
                 foreach (array_filter(array($base[$i], $variant)) as $unit) {
@@ -379,8 +412,14 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             }
             if (null !== $reason) { $reasons[$owner] = $reason; continue; }
             $scoped[] = (string) $owner;
+            $defined = $pageDefined;
             foreach ($localChanges as $i => $variant) $changes[$i][$owner] = $variant;
             foreach ($localInsertions as $i => $list) $insertions[$i][$owner] = $list;
+            foreach ($localSite as $i => $list) foreach ($list as $unit) {
+                if (isset($baseValues[$value($unit)])) continue;
+                $baseValues[$value($unit)] = true;
+                $siteInsertions[$i][] = $unit;
+            }
         }
         $output = array();
         $routeClass = static fn ($owner): string => \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($routeIdentities[$owner] ?? (string) $owner);
@@ -394,6 +433,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             foreach ($texts as $group) $output[] = $this->scopeRouteUnit($group['unit'], $group['routes'], true);
         };
         for ($i = 0; $i <= count($base); ++$i) {
+            foreach ($siteInsertions[$i] ?? array() as $unit) $output[] = $unit;
             $emitGroups($insertions[$i] ?? array());
             if ($i === count($base)) break;
             $variants = $changes[$i] ?? array();
@@ -403,40 +443,178 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         return array('stylesheet' => $this->serializeRuleUnits($output), 'scoped' => $scoped, 'reasons' => $reasons);
     }
 
+    /**
+     * The name a route-independent definition at-rule binds, or null for any
+     * other unit. Same-name definitions with different bodies conflict.
+     *
+     * @param array{context:list<string>,text:string,dedupable:bool} $unit
+     */
+    private function routeIndependentDefinition(array $unit): ?string
+    {
+        $rule = trim(CssSyntaxScanner::withoutComments($unit['text']));
+        if (!$this->isAtRule($rule)) return null;
+        $name = self::atRuleName($rule);
+        // A late @charset is ignored; dedupe it by text, never conflict.
+        if ('charset' === $name) return '@charset ' . $rule;
+        $brace = $this->nextRuleBoundary($rule, 0);
+        if (null === $brace || '{' !== $rule[$brace] || !str_ends_with($rule, '}')) return null;
+        $prelude = trim(substr($rule, strlen($name) + 1, $brace - strlen($name) - 1));
+        if (preg_match('/^(?:-[a-z]+-)?keyframes$/', $name)) return '@keyframes ' . trim($prelude, '"\'');
+        if (in_array($name, array('counter-style', 'property', 'font-palette-values'), true)) return '@' . $name . ' ' . $prelude;
+        if ('font-face' !== $name) return null;
+        $descriptors = array();
+        foreach (CssRuleAnalyzer::declarations(substr($rule, $brace + 1, -1), array('font-family', 'font-style', 'font-weight', 'font-stretch', 'unicode-range')) as $declaration) {
+            $descriptors[$declaration['name']] = strtolower(trim($declaration['value'], '"\' '));
+        }
+        ksort($descriptors);
+        return '@font-face ' . json_encode($descriptors);
+    }
+
+    /**
+     * Split a unit into the comments before it and the rule itself.
+     *
+     * @return array{0:string,1:string}
+     */
+    private static function splitLeadingComments(string $text): array
+    {
+        $offset = 0;
+        $length = strlen($text);
+        while (true) {
+            while ($offset < $length && CssSyntaxScanner::isCssWhitespace($text[$offset])) ++$offset;
+            if (!str_starts_with(substr($text, $offset, 2), '/*')) break;
+            $close = strpos($text, '*/', $offset + 2);
+            if (false === $close) break;
+            $offset = $close + 2;
+        }
+        return array(substr($text, 0, $offset), substr($text, $offset));
+    }
+
     /** @param array{context:list<string>,text:string,dedupable:bool} $unit @param list<string> $routes */
     private function scopeRouteUnit(array $unit, array $routes, bool $positive, ?string &$reason = null): array
     {
-        $brace = strpos($unit['text'], '{');
-        if (false === $brace || str_starts_with(ltrim($unit['text']), '@') || !$unit['dedupable']) { $reason = 'non-selector rule cannot be route scoped'; return $unit; }
-        $arms = self::splitSelectorList(trim(substr($unit['text'], 0, $brace)));
+        [$lead, $rule] = self::splitLeadingComments($unit['text']);
+        $brace = $this->nextRuleBoundary($rule, 0);
+        if (null === $brace || '{' !== $rule[$brace] || $this->isAtRule($rule) || !$unit['dedupable']) { $reason = 'non-selector rule cannot be route scoped'; return $unit; }
+        $arms = self::splitSelectorList(trim(CssSyntaxScanner::withoutComments(substr($rule, 0, $brace))));
         if (null === $arms) { $reason = 'unparseable selector list'; return $unit; }
         $scope = ':where(:root:' . ($positive ? 'is' : 'not') . '(' . implode(',', array_map(static fn (string $route): string => '.' . $route, $routes)) . '))';
+        $arms = self::scopeRouteArms($arms, $scope, $reason);
+        if (null === $arms) return $unit;
+        $unit['text'] = $lead . implode(',', $arms) . substr($rule, $brace);
+        return $unit;
+    }
+
+    /**
+     * Restrict each comment-free selector arm to documents whose root matches
+     * $scope, a zero-specificity `:where(:root…)`. The scope prefixes the arm
+     * as an ancestor; arms whose first compound can be the root also keep a
+     * copy with the scope merged into that compound. A first compound that
+     * tests the root inside `:is()`/`:where()` is scoped inside that argument.
+     *
+     * @param list<string> $arms
+     * @return list<string>|null
+     */
+    private static function scopeRouteArms(array $arms, string $scope, ?string &$reason): ?array
+    {
         $rootAlternatives = array();
         foreach ($arms as &$arm) {
             $arm = trim($arm);
             $tokens = CssSelectorTokenizer::tokenize($arm);
-            if (!$tokens['supported'] || str_contains($arm, '&') || str_contains($arm, '|') || str_contains($arm, '/*')) { $reason = 'unparseable or relative selector'; return $unit; }
-            if (preg_match('/::|:(?:before|after|first-line|first-letter)\b/i', $arm)) { $reason = 'pseudo-element selector'; return $unit; }
-            if (!(CssSelectorMatcher::parse($arm)['supported'] ?? false)) { $reason = 'unparseable selector'; return $unit; }
+            $structural = preg_replace('/"(?:[^"\\\\]|\\\\.)*"|\'(?:[^\'\\\\]|\\\\.)*\'|\[[^\]]*\]/s', '', $arm) ?? $arm;
+            if (!$tokens['supported'] || str_contains($structural, '&') || preg_match('/(?<!\|)\|(?!\|)/', $structural)) { $reason = 'unparseable or relative selector'; return null; }
             $first = $tokens['compounds'][0];
+            $end = $tokens['compound_spans'][0]['end'];
+            // A pseudo-element ends its compound: merge the scope before it.
+            $insert = $end;
+            if (1 === count($tokens['compounds'])) {
+                $pseudoElement = self::pseudoElementOffset($first);
+                if (null !== $pseudoElement) $insert = $pseudoElement;
+            }
             if (preg_match('/^(?:html(?=[.#:\[]|$)|:root(?=[.#:\[]|$))/i', $first)) {
-                $end = $tokens['compound_spans'][0]['end'];
-                $arm = substr($arm, 0, $end) . $scope . substr($arm, $end);
-            } else {
-                // Root tests hidden in functional selectors can match html too.
-                if (preg_match('/(?<![\w-])(?:html|:root)\b/i', $first)) { $reason = 'functional root selector'; return $unit; }
-                // Class/universal/attribute arms may themselves match html.
-                // Retain that match as well as descendants without adding weight.
-                if (preg_match('/^[*.#\[:]/', $first)) {
-                    $end = $tokens['compound_spans'][0]['end'];
-                    $rootAlternatives[] = substr($arm, 0, $end) . $scope . substr($arm, $end);
-                }
-                $arm = $scope . ' ' . $arm;
+                $arm = substr($arm, 0, $insert) . $scope . substr($arm, $insert);
+                continue;
+            }
+            $functional = preg_match('/(?<![\w-])(?:html|:root)\b/i', $first) ? self::scopeFunctionalRoot($first, $scope, $reason) : null;
+            if (null !== $reason) return null;
+            if (null !== $functional) {
+                $arm = $functional . substr($arm, $end);
+                continue;
+            }
+            // Class/universal/attribute/pseudo arms may themselves match html.
+            // Retain that match as well as descendants without adding weight.
+            if (preg_match('/^[*.#\[:]/', $first)) {
+                $rootAlternatives[] = substr($arm, 0, $insert) . $scope . substr($arm, $insert);
             }
+            $arm = $scope . ' ' . $arm;
         }
         unset($arm);
-        $unit['text'] = implode(',', array_merge($arms, $rootAlternatives)) . substr($unit['text'], $brace);
-        return $unit;
+        return array_merge($arms, $rootAlternatives);
+    }
+
+    /**
+     * Scope the first `:is()`/`:where()` argument list in $compound that
+     * tests the root, or null when the root test sits elsewhere (`:not()`,
+     * `:has()`), where an ancestor scope is already exact.
+     */
+    private static function scopeFunctionalRoot(string $compound, string $scope, ?string &$reason): ?string
+    {
+        if (!preg_match_all('/:(?:is|where|matches|-webkit-any|-moz-any)\(/i', $compound, $matches, PREG_OFFSET_CAPTURE)) return null;
+        foreach ($matches[0] as [$open, $offset]) {
+            $start = $offset + strlen($open);
+            if (0 !== self::parenDepth(substr($compound, 0, $offset))) continue;
+            $close = self::closingParen($compound, $start - 1);
+            if (null === $close) continue;
+            $argument = substr($compound, $start, $close - $start);
+            if (!preg_match('/(?<![\w-])(?:html|:root)\b/i', $argument)) continue;
+            $alternatives = self::splitSelectorList($argument);
+            if (null === $alternatives) { $reason = 'unparseable functional root selector'; return null; }
+            $scoped = self::scopeRouteArms($alternatives, $scope, $reason);
+            if (null === $scoped) return null;
+            return substr($compound, 0, $start) . implode(',', $scoped) . substr($compound, $close);
+        }
+        return null;
+    }
+
+    /** Byte offset of a top-level pseudo-element in a compound, if any. */
+    private static function pseudoElementOffset(string $compound): ?int
+    {
+        $state = CssSyntaxScanner::state();
+        $length = strlen($compound);
+        for ($offset = 0; $offset < $length; ) {
+            $topLevel = CssSyntaxScanner::isTopLevel($state);
+            $next = CssSyntaxScanner::consume($compound, $offset, $state);
+            if (null === $next) return null;
+            if ($topLevel && $next === $offset + 1 && ':' === $compound[$offset]) {
+                if (':' === ($compound[$offset + 1] ?? '') || preg_match('/\G:(?:before|after|first-line|first-letter)(?![\w-])/i', $compound, $_, 0, $offset)) return $offset;
+            }
+            $offset = $next;
+        }
+        return null;
+    }
+
+    private static function parenDepth(string $value): int
+    {
+        $state = CssSyntaxScanner::state();
+        $length = strlen($value);
+        for ($offset = 0; $offset < $length; ) {
+            $next = CssSyntaxScanner::consume($value, $offset, $state);
+            if (null === $next) return -1;
+            $offset = $next;
+        }
+        return $state['parens'];
+    }
+
+    private static function closingParen(string $value, int $open): ?int
+    {
+        $state = CssSyntaxScanner::state();
+        $length = strlen($value);
+        for ($offset = $open; $offset < $length; ) {
+            $next = CssSyntaxScanner::consume($value, $offset, $state);
+            if (null === $next) return null;
+            if ($next === $offset + 1 && ')' === $value[$offset] && 0 === $state['parens'] && '' === $state['quote'] && !$state['comment']) return $offset;
+            $offset = $next;
+        }
+        return null;
     }
 
     /**
diff --git a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
index 26c193fc0..a054f60e1 100644
--- a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
+++ b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
@@ -24,7 +24,7 @@ public static function metadataFromHtml(string $html, string $routeIdentity = ''
         return array('root_attributes' => $attributes['html'], 'body_attributes' => $attributes['body']);
     }
 
-    /** Stable document compilation identity, shared by CSS and root reconciliation. */
+    /** Stable per-document route class from the compile-time source identity, used by CSS and the root registry alike. */
     public static function routeClass(string $identity): string
     {
         return 'be-route-' . substr(hash('sha256', $identity), 0, 12);
diff --git a/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php b/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php
index d92ace297..9ef531467 100644
--- a/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php
+++ b/php-transformer/src/WordPressSitePlan/WordPressSitePlan.php
@@ -453,28 +453,6 @@ public function fromCompilerInput(array $data, WordPressSitePlanInput $input): a
         unset($template);
         $runtimeDeclarations = $this->canonicalEntityBindings($runtimeDeclarations, $runtimeEntityRecords, $references, $routeMap, $pages, $bindingTemplates, $paginationOwners);
         $pages = $this->pageHierarchy($pages, $routeMap);
-        // Compilation knows source identities; route reconciliation finalizes
-        // them here. Remap the static CSS and the existing root registry together.
-        $routeClasses = array();
-        foreach ($pages as &$page) {
-            if (!empty($page['synthetic'])) continue;
-            $sourceClass = DocumentRootContext::routeClass(self::identity('page', $page['source_path'], $page['source_path']));
-            $routeClass = DocumentRootContext::routeClass($page['reconciliation_identity']);
-            $routeClasses[$sourceClass] = $routeClass;
-            $root = $page['document_metadata']['root_attributes'] ?? array();
-            $root['class'] = trim(str_replace($sourceClass, $routeClass, $root['class'] ?? ''));
-            if (!in_array($routeClass, explode(' ', $root['class']), true)) $root['class'] = trim($root['class'] . ' ' . $routeClass);
-            $page['document_metadata']['root_attributes'] = $root;
-        }
-        unset($page);
-        foreach ($assets as &$asset) {
-            if ('css' !== ($asset['kind'] ?? '') || !is_string($asset['content'] ?? null)) continue;
-            $asset['content'] = strtr($asset['content'], $routeClasses);
-            $asset['bytes'] = strlen($asset['content']);
-            $asset['hash'] = hash('sha256', $asset['content']);
-            $asset['content_hash'] = self::contentHash($asset['content']);
-        }
-        unset($asset);
         $templateOwners = array();
         foreach ($pages as $page) if (empty($page['entrypoint'])) $templateOwners[self::routeTemplateKey($page)][] = $page['source_path'];
         // Colliding terminal slugs get a readable route-qualified template
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index 3bd77c094..2dde3e30e 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -331,9 +331,9 @@
 $assert(1 === count($routeAssets) && !str_contains(implode('', array_column($routePlan['assets'], 'source_path')), '.page-'), 'constant-arm variants, dropped rules and page-only rules need no copies');
 $classes = array();
 foreach ($routePlan['pages'] as $row) {
-    $class = Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($row['reconciliation_identity']);
+    $class = Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass(WordPressSitePlan::identity('page', $row['source_path'], $row['source_path']));
     $classes[$row['source_path']] = $class;
-    $assert(in_array($class, explode(' ', $row['document_metadata']['root_attributes']['class'] ?? ''), true), 'document root carries its reconciliation route class: ' . $row['source_path']);
+    $assert(in_array($class, explode(' ', $row['document_metadata']['root_attributes']['class'] ?? ''), true), 'document root carries its source route class: ' . $row['source_path']);
     $functions = array_column($routePlan['writes'], null, 'target_path')['functions.php']['payload']['data'];
     $assert(str_contains($functions, $class) && str_contains($functions, 'blocksEngineDocumentContext') && str_contains($functions, 'language_attributes'), 'the same root registry reaches front end and editor: ' . $row['source_path']);
 }
@@ -354,7 +354,7 @@
 
 // Each route is compared with its independent public compilation, including
 // editor wrapper selectors. Apply exactly the root attributes the theme uses.
-$probeStyles = static function (array $plan, array $row, bool $editor): array {
+$probeStyles = static function (array $plan, array $row, bool $editor, array $properties = array('color', 'background-color', 'padding-top', 'border-top-width')): array {
     $assets = array_column($plan['assets'], null, 'target_path');
     $css = '';
     foreach (WordPressSitePlan::documentStylesheets($plan, $row['source_path']) as $sheet) $css .= "\n" . ($assets[$sheet['target_path']]['content'] ?? '');
@@ -369,7 +369,7 @@
         if ('img' === $element->tagName) {
             $styles[] = $cascade->resolve($element, array('width', 'max-width', 'display'), array());
         } elseif ('Probe' === trim($element->textContent) && in_array($element->tagName, array('p', 'span', 'a', 'button'), true)) {
-            $styles[] = $cascade->resolve($element, array('color', 'background-color', 'padding-top', 'border-top-width'), array());
+            $styles[] = $cascade->resolve($element, $properties, array());
         }
     }
     return $styles;
@@ -398,7 +398,9 @@
 $copies = array_filter($fallbackAssets, static fn (array $asset): bool => str_contains($asset['source_path'], '.page-'));
 $assert(array() !== $copies && array() === array_filter($copies, static fn (array $asset): bool => str_starts_with($asset['source_path'], 'base.page-')), 'fallback is restricted to the unsafe sheet');
 foreach (array(
-    array('.x::before{color:red}', '.y::before{color:red}', 'pseudo-element'),
+    array('@keyframes k{to{opacity:0}}.x{color:red}', '.x{color:red}@keyframes k{to{opacity:1}}', 'conflicting @keyframes k'),
+    array('@font-face{font-family:"F";src:url(a.woff2)}.x{color:red}', '.x{color:red}@font-face{font-family:F;src:url(b.woff2)}', 'conflicting @font-face'),
+    array('.x{color:red}', '@page{margin:0}.x{color:red}', 'non-selector'),
     array('.x{color:red}', '& .x{color:red}', 'relative'),
     array('.x{color:red}', '.x{color:red', 'malformed'),
 ) as [$base, $variant, $reason]) {
@@ -406,6 +408,49 @@
     $assert(array() === $routed['scoped'] && str_contains($routed['reasons']['unsafe'] ?? '', $reason), 'unsafe route falls back: ' . $reason);
 }
 
+// Real-site shapes that used to send a whole page x stylesheet pair back to
+// a page copy. Each page's `button` arms are rewritten differently from the
+// span page's, so the feature rides in a route variant or a dropped rule. The
+// button page's extra `!important` override aligns with `.tail`, so that page
+// drops `@keyframes` at its source position and re-adds it after `.tail`.
+$probeProperties = array('color', 'background-color', 'padding-top', 'border-top-width', 'margin-top', 'animation-name');
+foreach (array(
+    'leading comment' => '/*# sourceMappingURL=x.map */ /*! Lib | a & b {} */ button,.sample{padding:7px}/* #global {color:red} */ .sample{margin-top:3px}',
+    'interactive pseudo-classes' => 'button,select:disabled,#lien .content_site:hover .actions,.input-group:not(.has-validation)>.dropdown-toggle:nth-last-child(n+3){border-top-width:4px}',
+    'pseudo-element arm' => 'button,.sample::-webkit-date-and-time-value,::-moz-focus-inner,[list]:not([type=date])::-webkit-calendar-picker-indicator,.sample::before{padding:5px}',
+    'keyframes dropped on one page' => '/*# sourceMappingURL=x.map */@charset "UTF-8";button{padding:7px}@keyframes spinner-border{to{transform:rotate(1turn)}}.tail{padding:7px!important}.sample{animation:spinner-border 1s}',
+    'functional root arm' => ':where(:root .editor-styles-wrapper) button,:where(:root .editor-styles-wrapper) .sample[data-empty="true"],:where(:root) .sample{background-color:#abcdef}',
+) as $label => $variantCss) {
+    $variantSheet = array('path' => 'base.css', 'kind' => 'css', 'content' => $variantCss . '.sample{color:#123456}');
+    $variantFiles = array_slice($routeFiles, 0, 3);
+    $variantResult = (new ArtifactCompiler())->compile(array('entrypoint' => 'a.html', 'files' => array_merge($variantFiles, array($variantSheet))))->toArray();
+    $variantPlan = $variantResult['source_reports']['wordpress_site_plan'];
+    $variantFallbacks = array_values(array_filter($variantResult['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '')));
+    $assert(array() === $variantFallbacks, $label . ': every route is scoped in place', json_encode(array_column($variantFallbacks, 'message')));
+    $assert(!str_contains(implode('', array_column($variantPlan['assets'], 'source_path')), '.page-'), $label . ': no page-copy assets');
+    $variantText = implode('', $authoredAssets($variantPlan));
+    $assert(str_contains($variantText, ':where(:root:'), $label . ': the shared sheet carries route scopes', $variantText);
+    foreach ($variantPlan['pages'] as $row) {
+        $file = array_column($variantFiles, null, 'path')[$row['source_path']];
+        $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $file['path'], 'files' => array($file, $variantSheet)))->toArray()['source_reports']['wordpress_site_plan'];
+        foreach (array(false, true) as $editor) {
+            $actual = $probeStyles($variantPlan, $row, $editor, $probeProperties);
+            $expected = $probeStyles($alone, $alone['pages'][0], $editor, $probeProperties);
+            $assert(array() !== $actual && $expected === $actual, $label . ': computed-style parity ' . $row['source_path'] . ($editor ? ' editor' : ' front end'), json_encode(array($expected, $actual)));
+        }
+    }
+    if ('keyframes dropped on one page' === $label) {
+        $assert(1 === substr_count($variantText, '@keyframes spinner-border') && !preg_match('/:root:[^{]*\{[^{}]*@keyframes/', $variantText), $label . ': the definition is emitted once, unscoped', $variantText);
+    }
+    if ('functional root arm' === $label) {
+        $assert(str_contains($variantText, ':where(:root:where(:root:is(') || str_contains($variantText, ':where(:root:where(:root:not('), $label . ': the route class merges into the inner root compound', $variantText);
+    }
+    if ('leading comment' === $label) {
+        $assert(str_contains($variantText, '/*! Lib | a & b {} */'), $label . ': leading comments are kept inert', $variantText);
+    }
+    echo 'Variant fixture (' . $label . ') author CSS: ' . strlen($variantText) . ' bytes in ' . count($authoredAssets($variantPlan)) . " assets\n";
+}
+
 // Identical shared units are one fold match, rewritten at every occurrence.
 $marker = 'blocks-engine-control-0123456789ab-0';
 $unit = '.group>.btn{z-index:1}';

From 56047163740d16bbdf091ac1e375949888934a5f Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Mon, 26 Oct 2026 20:18:27 -0400
Subject: [PATCH 03/10] Fold page-local marker variants into shared rules
 before route scoping

---
 .../src/ArtifactCompiler/ArtifactCompiler.php |  45 +++++-
 .../src/Css/CssSelectorListCompactor.php      | 153 +++++++++++++++++-
 .../src/Css/CssStylesheetTransformer.php      | 153 +++++++++++++++---
 .../shared-author-stylesheet-scaling.php      | 104 ++++++++++++
 .../tests/unit/css-stylesheet-transformer.php |  14 ++
 5 files changed, 434 insertions(+), 35 deletions(-)

diff --git a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
index 8584036ea..1fe648db9 100644
--- a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
+++ b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
@@ -22,6 +22,7 @@
 use Automattic\BlocksEngine\PhpTransformer\Css\AdminBarAccommodation;
 use Automattic\BlocksEngine\PhpTransformer\Css\TemplatePartStickyContainment;
 use Automattic\BlocksEngine\PhpTransformer\Css\AuthorCascadeLayerOrder;
+use Automattic\BlocksEngine\PhpTransformer\Css\CssSelectorListCompactor;
 use Automattic\BlocksEngine\PhpTransformer\Css\CssStylesheetTransformer;
 use Automattic\BlocksEngine\PhpTransformer\Css\CssValueSplitter;
 use Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\Style\CssStylesheetChunker;
@@ -50,6 +51,20 @@ final class ArtifactCompiler
 
     public const MAX_PLAN_DIGEST_DEPTH = 512;
 
+    /**
+     * Route-scoped variants live in the one sheet every route downloads. When
+     * the bytes they add (scoped variants and page-only rules, plus the growth
+     * of shared rules that exclude routes) exceed this factor times the
+     * largest page copy (`<sheet>.page-<hash>.css`, what one page would load
+     * on top of the shared sheet), that stylesheet falls back to page copies
+     * with an `author_stylesheet_route_scope_fallback` diagnostic. A visitor
+     * then never loads more than about three times the extra CSS of a copy.
+     */
+    public const ROUTE_SCOPE_BYTES_FACTOR = 3;
+
+    /** Route-scoping overhead always allowed, e.g. for routes that only drop rules (their copy is empty). */
+    public const ROUTE_SCOPE_BYTES_FLOOR = 8192;
+
     public const INPUT_SCHEMA = 'blocks-engine/php-transformer/site-artifact/v1';
     // Plans are a published v1 transport contract. Receipts are additive: v2
     // carries terminal reductions while v1 receipts remain composable.
@@ -2639,17 +2654,35 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             $sharedStylesheets = $folded['shared'];
             $fallbackSharedStylesheets = $sharedStylesheets;
             foreach ($pages as $owner => $_) $pages[$owner] = $folded['pages'][$owner] ?? $pages[$owner];
-            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $folded['pages'], $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null);
-            $sharedStylesheets = array($routed['stylesheet']);
-            foreach ($routed['scoped'] as $owner) unset($pages[$owner]);
+            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $folded['pages'], $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup);
+            // Folded rules carry expanded marker arms; recompact the serialized
+            // sheet, merging marker arms into `:where()` lists across routes.
+            $recompact = static fn (string $css): string => count($folded['pages']) > 1 ? CssSelectorListCompactor::stylesheet($css, true) : $css;
+            // Bound what route scoping adds to the sheet every route loads by
+            // what the largest page copy would have added to one page. Both
+            // are measured before compaction.
+            $largestCopy = 0;
+            if (0 < $routed['route_bytes']) {
+                foreach ($routed['scoped'] as $owner) {
+                    $largestCopy = max($largestCopy, strlen($transformer->rulesAbsentFrom($folded['pages'][$owner] ?? array(), $fallbackSharedStylesheets, true)));
+                }
+            }
+            $routeBudget = max(self::ROUTE_SCOPE_BYTES_FACTOR * $largestCopy, self::ROUTE_SCOPE_BYTES_FLOOR);
+            if (array() !== $routed['scoped'] && $routed['route_bytes'] > $routeBudget) {
+                $projectionDiagnostics[] = $this->diagnostic('author_stylesheet_route_scope_fallback', 'warning', sprintf('%s: route-scoped variants add %d bytes to every route, over %d (%dx the largest %d-byte page copy); emitting page copies', $path, $routed['route_bytes'], $routeBudget, self::ROUTE_SCOPE_BYTES_FACTOR, $largestCopy));
+                foreach ($routed['scoped'] as $owner) $pages[$owner] ??= $folded['pages'][$owner] ?? array();
+                $routed['stylesheet'] = implode("\n", $fallbackSharedStylesheets);
+            } else {
+                foreach ($routed['scoped'] as $owner) unset($pages[$owner]);
+            }
             foreach ($routed['reasons'] as $owner => $reason) {
                 $projectionDiagnostics[] = $this->diagnostic('author_stylesheet_route_scope_fallback', 'warning', $path . ' (' . $owner . '): ' . $reason);
             }
-            $projectedFile = $this->projectedStylesheetFile($file, $path, implode("\n", array_merge($preambles, array( $transformer->concatenateWithoutRedundantRules($sharedStylesheets) ))));
+            $projectedFile = $this->projectedStylesheetFile($file, $path, implode("\n", array_merge($preambles, array( $recompact($transformer->concatenateWithoutRedundantRules(array($routed['stylesheet']))) ))));
             $projectedFile['metadata']['native_selectors'] = array_keys($native);
             $output[] = $projectedFile;
             foreach ( $pages as $owner => $pageStylesheets ) {
-                $delta = $transformer->rulesAbsentFrom($pageStylesheets, $fallbackSharedStylesheets, true);
+                $delta = $recompact($transformer->rulesAbsentFrom($pageStylesheets, $fallbackSharedStylesheets, true));
                 if ( '' === trim($delta) ) {
                     continue;
                 }
@@ -2657,7 +2690,7 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
                 $pageFile['metadata']['compilation'] = array( 'scope' => 'page', 'id' => (string) $owner );
                 $pageFile['metadata']['page_stylesheet_of'] = $path;
                 $pageNative = $native;
-                foreach ($originalPages[$owner] as $content) {
+                foreach ($originalPages[$owner] ?? array() as $content) {
                     $pageNative += $nativeByContent[$path][$content] ?? array();
                 }
                 $pageFile['metadata']['native_selectors'] = array_keys($pageNative);
diff --git a/php-transformer/src/Css/CssSelectorListCompactor.php b/php-transformer/src/Css/CssSelectorListCompactor.php
index aad4f6331..242f815e8 100644
--- a/php-transformer/src/Css/CssSelectorListCompactor.php
+++ b/php-transformer/src/Css/CssSelectorListCompactor.php
@@ -10,13 +10,18 @@ final class CssSelectorListCompactor
     private static array $stylesheets = array();
     private static int $cachedBytes = 0;
 
-    public static function stylesheet(string $css): string
+    /**
+     * @param bool $markerLists Also merge arms that differ only by one
+     *     page-local marker class in a `:where()` into `:where(.m1,.m2,…)`.
+     *     Used for the site-wide sheet, where routes contribute marker arms.
+     */
+    public static function stylesheet(string $css, bool $markerLists = false): string
     {
-        $key = hash('sha256', $css);
+        $key = hash('sha256', $css) . ($markerLists ? ':m' : '');
         if ( isset(self::$stylesheets[ $key ]) ) {
             return self::$stylesheets[ $key ];
         }
-        $result = self::compactStylesheet($css);
+        $result = self::compactStylesheet($css, $markerLists);
         if ( count(self::$stylesheets) >= 128 || self::$cachedBytes + strlen($result) > 2097152 ) {
             self::$stylesheets = array();
             self::$cachedBytes = 0;
@@ -28,12 +33,150 @@ public static function stylesheet(string $css): string
         return $result;
     }
 
-    private static function compactStylesheet(string $css): string
+    private static function compactStylesheet(string $css, bool $markerLists = false): string
     {
         if ( ! str_contains($css, ',') || preg_match('/@namespace\b/i', $css) ) {
             return $css;
         }
-        return (new CssStylesheetTransformer())->transform($css, static fn(string $prelude): string => self::compact($prelude));
+        return (new CssStylesheetTransformer())->transform($css, static fn(string $prelude): string => self::compact($markerLists ? self::mergeMarkerLists($prelude) : $prelude));
+    }
+
+    /**
+     * Merge arms identical but for the page-local marker class inside one
+     * top-level `:where(.marker)`: `X:where(.m1)Y,X:where(.m2)Y` becomes
+     * `X:where(.m1,.m2)Y`. `:where()` weighs nothing and matches when any
+     * argument does, and every argument is a plain class, so the list is
+     * equivalent. expand() splits such lists back into arms.
+     */
+    public static function mergeMarkerLists(string $prelude): string
+    {
+        if ( ! str_contains($prelude, ',') || ! preg_match(self::PAGE_MARKER, $prelude) ) {
+            return $prelude;
+        }
+        $arms = CssStylesheetTransformer::splitSelectorList($prelude);
+        if ( null === $arms || count($arms) < 2 ) {
+            return $prelude;
+        }
+        $groups = array();
+        $order = array();
+        foreach ( $arms as $index => $arm ) {
+            $arm = trim($arm);
+            $key = "\1" . $index;
+            $markers = array();
+            if ( ! preg_match('/::|:(?:before|after|first-line|first-letter)\b/i', $arm) && ! str_contains($arm, '/*') && false === strpbrk($arm, '\\"\'') && preg_match_all(self::MARKER_WHERE, $arm, $matches, PREG_OFFSET_CAPTURE) ) {
+                foreach ( $matches[0] as $index2 => [$text, $offset] ) {
+                    $before = substr($arm, 0, $offset);
+                    if ( substr_count($before, '(') !== substr_count($before, ')') ) {
+                        continue;
+                    }
+                    $key = substr($arm, 0, $offset) . "\0" . substr($arm, $offset + strlen($text));
+                    $markers = explode(',', $matches[1][ $index2 ][0]);
+                    break;
+                }
+            }
+            if ( ! isset($groups[ $key ]) ) {
+                $order[] = $key;
+                $groups[ $key ] = array('arm' => $arm, 'markers' => array());
+            }
+            foreach ( $markers as $marker ) {
+                $groups[ $key ]['markers'][ $marker ] = true;
+            }
+        }
+        if ( count($order) === count($arms) ) {
+            return $prelude;
+        }
+        $output = array();
+        foreach ( $order as $key ) {
+            $group = $groups[ $key ];
+            if ( array() === $group['markers'] ) {
+                $output[] = $group['arm'];
+                continue;
+            }
+            [$prefix, $suffix] = explode("\0", $key, 2);
+            $output[] = $prefix . ':where(' . implode(',', array_keys($group['markers'])) . ')' . $suffix;
+        }
+        return implode(',', $output);
+    }
+
+    /** A `:where()` whose arguments are page-local marker classes only. */
+    private const MARKER_WHERE = '/:where\(((?:\.blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+,)*\.blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+)\)/';
+
+    /**
+     * Inverse of compact() for page-local marker lists: expand every whole
+     * compound `:is(m1,m2,…)` whose alternatives share one specificity in the
+     * strict grammar, and at least one names a page-local block marker, back
+     * into one arm per alternative. Equal specificity and strict alternatives
+     * make the arm list cascade-equivalent to the packed form, so two pages'
+     * projections of one source rule align arm by arm before reconciliation.
+     */
+    public static function expand(string $prelude): string
+    {
+        if ( ! preg_match(self::PAGE_MARKER, $prelude) ) {
+            return $prelude;
+        }
+        $arms = CssStylesheetTransformer::splitSelectorList($prelude);
+        if ( null === $arms ) {
+            return $prelude;
+        }
+        $output = array();
+        $changed = false;
+        while ( array() !== $arms ) {
+            $arm = trim((string) array_shift($arms));
+            $expanded = self::expandArm($arm);
+            if ( null === $expanded ) {
+                $output[] = $arm;
+                continue;
+            }
+            $changed = true;
+            array_unshift($arms, ...$expanded);
+        }
+        return $changed ? implode(',', $output) : $prelude;
+    }
+
+    /** Page-local block marker class (`blocks-engine-<kind>-<hash>-<n>`). */
+    public const PAGE_MARKER = '/(?<![a-zA-Z0-9_-])blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+(?![a-zA-Z0-9_-])/';
+
+    /** @return list<string>|null */
+    private static function expandArm(string $arm): ?array
+    {
+        if ( false === strpbrk($arm, '\\"\'') && preg_match_all(self::MARKER_WHERE, $arm, $matches, PREG_OFFSET_CAPTURE) ) {
+            foreach ( $matches[0] as $index => [$text, $offset] ) {
+                $before = substr($arm, 0, $offset);
+                if ( ! str_contains($matches[1][ $index ][0], ',') || substr_count($before, '(') !== substr_count($before, ')') ) {
+                    continue;
+                }
+                $suffix = substr($arm, $offset + strlen($text));
+                return array_map(static fn(string $marker): string => $before . ':where(' . $marker . ')' . $suffix, explode(',', $matches[1][ $index ][0]));
+            }
+        }
+        if ( ! str_contains($arm, ':is(') ) {
+            return null;
+        }
+        $tokens = CssSelectorTokenizer::tokenize($arm);
+        if ( ! $tokens['supported'] ) {
+            return null;
+        }
+        foreach ( $tokens['compound_spans'] as $position => $span ) {
+            $compound = $tokens['compounds'][ $position ];
+            if ( ! str_starts_with($compound, ':is(') || ! str_ends_with($compound, ')') || ! preg_match(self::PAGE_MARKER, $compound) ) {
+                continue;
+            }
+            $alternatives = CssStylesheetTransformer::splitSelectorList(substr($compound, 4, -1));
+            if ( null === $alternatives || count($alternatives) < 2 ) {
+                continue;
+            }
+            $weights = array();
+            foreach ( $alternatives as $alternative ) {
+                $weights[ (string) self::plainCompoundWeight(trim($alternative)) ] = true;
+            }
+            if ( 1 !== count($weights) || isset($weights['']) ) {
+                continue;
+            }
+            $prefix = substr($arm, 0, $span['start']);
+            $suffix = substr($arm, $span['end']);
+            return array_map(static fn(string $alternative): string => $prefix . trim($alternative) . $suffix, $alternatives);
+        }
+        return null;
     }
 
     public static function compact(string $prelude): string
diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 19b287313..166d32535 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -227,6 +227,14 @@ public function rulesAbsentFrom(array $stylesheets, array $present, bool $preser
      * is deliberately unsafe. Return equivalent page rules using the union so
      * rulesAbsentFrom() can remove them without moving a cascade suffix.
      *
+     * Reconciliation runs on uncompacted arms: per-page projections pack
+     * marker arms into `P:is(:where(.m1),…)S`, so every compared rule is
+     * expanded back to one arm per marker (rules left as they are keep their
+     * text). A marker arm is any arm with a top-level positive
+     * `:where(.marker)`, wherever it sits in the selector: it cannot match an
+     * element that does not carry the marker. Folded rules
+     * are uncompacted; the caller recompacts the final serialized sheet.
+     *
      * @param list<string> $shared
      * @param array<string, list<string>> $pages All consuming page projections.
      * @param array<string, string> $markup Canonical markup by page owner.
@@ -238,25 +246,14 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
         foreach (array_merge($shared, ...array_values($pages)) as $css) {
             if (!$this->isWellFormedStylesheet($css)) return $unchanged;
         }
-        $carriers = array();
-        foreach ($markup as $owner => $html) {
-            preg_match_all('/(?<![a-zA-Z0-9_-])blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+(?![a-zA-Z0-9_-])/', $html, $matches);
-            foreach (array_unique($matches[0]) as $marker) $carriers[$marker][$owner] = true;
-        }
-        // Only accept a literal marker in a positive :where() compound. This
-        // excludes marker negations, constant role classes, escapes and selector-list
-        // alternatives that could match without carrying the identity marker.
-        $markerOf = static function (string $arm): ?string {
-            $positive = preg_replace('/:not\((?:\.be-c-[0-9]+|be-t-[0-9]+)\)/', '', $arm);
-            if (str_contains($arm, '/*') || preg_match('/:(?:not|is|has)\s*\(/i', $positive)) return null;
-            return preg_match('/^:where\(\.(blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+)\)(?=$|[\s>+~.:#\[])/', $arm, $match) ? $match[1] : null;
-        };
+        $carriers = self::markerCarriers($markup);
+        $markerOf = static fn (string $arm): ?string => self::positiveMarker($arm);
         $describe = static function (array $unit) use ($markerOf): ?array {
             $brace = strpos($unit['text'], '{');
             if (!$unit['dedupable'] || false === $brace) return null;
             $prelude = trim(substr($unit['text'], 0, $brace));
             if (str_starts_with($prelude, '@')) return null;
-            $arms = self::splitSelectorList($prelude);
+            $arms = self::splitSelectorList(CssSelectorListCompactor::expand($prelude));
             if (null === $arms) return null;
             $arms = array_map('trim', $arms);
             $constant = array_values(array_filter($arms, static fn (string $arm): bool => null === $markerOf($arm)));
@@ -306,14 +303,22 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
                 }
                 if ($safe) $union[] = $arm;
             }
-            if ($union === $base['arms']) continue;
-            foreach ($matches as $match) $sharedUnits[$match]['text'] = implode(',', $union) . substr($sharedUnits[$match]['text'], $base['brace']);
+            if ($union !== $base['arms']) {
+                foreach ($matches as $match) $sharedUnits[$match]['text'] = implode(',', $union) . substr($sharedUnits[$match]['text'], $base['brace']);
+            }
+            // A page rule becomes the shared text when the shared rule only
+            // adds arms whose marker the page does not carry, whatever the
+            // packing of either side.
             foreach ($pageIndex as $owner => $rules) {
+                if (!isset($markup[$owner])) continue;
                 foreach ($rules[$key] ?? array() as $j) {
+                    if ($pageUnits[$owner][$j]['text'] === $sharedUnits[$i]['text']) continue;
                     $description = $describe($pageUnits[$owner][$j]);
-                    if (array() === array_diff($description['arms'], $union)) {
-                        $pageUnits[$owner][$j]['text'] = $sharedUnits[$i]['text'];
+                    if (array() !== array_diff($description['arms'], $union)) continue;
+                    foreach (array_diff($union, $description['arms']) as $arm) {
+                        if (isset($carriers[(string) $markerOf($arm)][$owner])) continue 2;
                     }
+                    $pageUnits[$owner][$j]['text'] = $sharedUnits[$i]['text'];
                 }
             }
         }
@@ -322,6 +327,39 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
         return $result;
     }
 
+    /**
+     * Owners whose markup carries each page-local marker.
+     *
+     * @param array<string,string> $markup
+     * @return array<string,array<string,true>>
+     */
+    private static function markerCarriers(array $markup): array
+    {
+        $carriers = array();
+        foreach ($markup as $owner => $html) {
+            preg_match_all(CssSelectorListCompactor::PAGE_MARKER, $html, $matches);
+            foreach (array_unique($matches[0]) as $marker) $carriers[$marker][$owner] = true;
+        }
+        return $carriers;
+    }
+
+    /**
+     * The page-local marker an arm requires: a literal `:where(.marker)` at
+     * selector top level, outside every `:not()`/`:is()`/`:has()` argument.
+     * Comments, escapes and strings are refused, so a marker negation, a
+     * selector-list alternative or quoted text never counts as a requirement.
+     */
+    private static function positiveMarker(string $arm): ?string
+    {
+        if (str_contains($arm, '/*') || strpbrk($arm, '\\"\'') !== false) return null;
+        if (!preg_match_all('/:where\(\.(blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+)\)/', $arm, $matches, PREG_OFFSET_CAPTURE)) return null;
+        foreach ($matches[0] as $index => [, $offset]) {
+            $before = substr($arm, 0, $offset);
+            if (substr_count($before, '(') === substr_count($before, ')') && substr_count($before, '[') === substr_count($before, ']')) return $matches[1][$index][0];
+        }
+        return null;
+    }
+
     /**
      * Reconcile ordered page projections at the source cascade position.
      * Unsupported pages are returned separately for the legacy copy path.
@@ -333,11 +371,39 @@ public function foldPositiveMarkerArms(array $shared, array $pages, array $marku
      *
      * @param list<string> $shared
      * @param array<string,list<string>> $pages
+     * Rules whose every arm requires a page-local marker (see
+     * positiveMarker()) need no route scope when $markup proves where the
+     * markers live: a route that drops such a rule and carries none of its
+     * markers loses nothing, and a route-only insertion whose markers only
+     * that route carries is emitted unscoped, merged by arm union with the
+     * other routes' insertions of the same rule at the same position.
+     *
      * @param array<string,string> $routeIdentities Route identities by compilation owner.
-     * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>}
+     * @param array<string,string> $markup Canonical markup by compilation owner.
+     * `route_bytes` counts what route scoping adds: positively scoped rules
+     * plus the growth of negatively scoped shared rules, before compaction.
+     *
+     * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>, route_bytes:int}
      */
-    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null): array
+    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array()): array
     {
+        $carriers = self::markerCarriers($markup);
+        // Owners that can carry every marker the unit's arms require, or null
+        // when some arm can match without a page-local marker.
+        $markerOwners = function (array $unit) use ($carriers): ?array {
+            [, $rule] = self::splitLeadingComments($unit['text']);
+            $brace = $this->nextRuleBoundary($rule, 0);
+            if (null === $brace || '{' !== $rule[$brace] || $this->isAtRule($rule) || !$unit['dedupable']) return null;
+            $arms = self::splitSelectorList(CssSelectorListCompactor::expand(substr($rule, 0, $brace)));
+            if (null === $arms) return null;
+            $owners = array();
+            foreach ($arms as $arm) {
+                $marker = self::positiveMarker(trim($arm));
+                if (null === $marker) return null;
+                $owners += $carriers[$marker] ?? array();
+            }
+            return $owners;
+        };
         $base = array();
         foreach ($shared as $css) $this->collectRuleUnits($css, array(), $base);
         $normal = static fn (array $unit): string => trim(CssSyntaxScanner::withoutComments($unit['text']));
@@ -384,6 +450,13 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                 $cursor = $match + 1;
             }
             while ($cursor < count($base)) $localChanges[$cursor++] = null;
+            $known = isset($markup[$owner]);
+            foreach ($localChanges as $i => $variant) {
+                if (null !== $variant || !$known) continue;
+                $owners = $markerOwners($base[$i]);
+                // Dropping a rule this route cannot match changes nothing.
+                if (null !== $owners && !isset($owners[$owner])) unset($localChanges[$i]);
+            }
             // A definition the route drops or adds stays site-wide, unless the
             // route would then see a different body under the same name.
             $pageDefined = $defined;
@@ -407,7 +480,12 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                     $this->scopeRouteUnit($unit, array('be-route-probe'), true, $reason);
                 }
             }
-            foreach ($localInsertions as $list) foreach ($list as $unit) {
+            foreach ($localInsertions as $i => $list) foreach ($list as $ordinal => $unit) {
+                $owners = $known ? $markerOwners($unit) : null;
+                if (null !== $owners && array((string) $owner) === array_map('strval', array_keys($owners))) {
+                    $localInsertions[$i][$ordinal]['own_markers'] = true;
+                    continue;
+                }
                 $this->scopeRouteUnit($unit, array('be-route-probe'), true, $reason);
             }
             if (null !== $reason) { $reasons[$owner] = $reason; continue; }
@@ -423,14 +501,40 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         }
         $output = array();
         $routeClass = static fn ($owner): string => \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($routeIdentities[$owner] ?? (string) $owner);
-        $emitGroups = function (array $groups) use (&$output, $routeClass): void {
+        $routeBytes = 0;
+        $emitGroups = function (array $groups) use (&$output, &$routeBytes, $routeClass): void {
             $texts = array();
             foreach ($groups as $owner => $units) foreach ($units as $ordinal => $unit) {
+                if (!empty($unit['own_markers'])) {
+                    // Inert off its route: union the arms, no route scope.
+                    unset($unit['own_markers']);
+                    [$lead, $rule] = self::splitLeadingComments($unit['text']);
+                    $brace = (int) $this->nextRuleBoundary($rule, 0);
+                    $groupKey = serialize(array($ordinal, $unit['context'], substr($rule, $brace), 'union'));
+                    $texts[$groupKey]['ordinal'] = $ordinal;
+                    $texts[$groupKey]['unit'] ??= $unit;
+                    $texts[$groupKey]['lead'] ??= $lead;
+                    $texts[$groupKey]['body'] = substr($rule, $brace);
+                    foreach ((array) self::splitSelectorList(CssSelectorListCompactor::expand(substr($rule, 0, $brace))) as $arm) $texts[$groupKey]['arms'][trim($arm)] = true;
+                    continue;
+                }
                 $groupKey = serialize(array($ordinal, $unit));
+                $texts[$groupKey]['ordinal'] = $ordinal;
                 $texts[$groupKey]['unit'] = $unit;
                 $texts[$groupKey]['routes'][] = $routeClass($owner);
             }
-            foreach ($texts as $group) $output[] = $this->scopeRouteUnit($group['unit'], $group['routes'], true);
+            // Each route has one unit per ordinal; ordinal order keeps every
+            // route's own insertions in its source order.
+            uasort($texts, static fn (array $a, array $b): int => $a['ordinal'] <=> $b['ordinal']);
+            foreach ($texts as $group) {
+                if (isset($group['arms'])) {
+                    $group['unit']['text'] = $group['lead'] . implode(',', array_keys($group['arms'])) . $group['body'];
+                    $output[] = $group['unit'];
+                    continue;
+                }
+                $output[] = $this->scopeRouteUnit($group['unit'], $group['routes'], true);
+                $routeBytes += strlen(end($output)['text']);
+            }
         };
         for ($i = 0; $i <= count($base); ++$i) {
             foreach ($siteInsertions[$i] ?? array() as $unit) $output[] = $unit;
@@ -438,9 +542,10 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             if ($i === count($base)) break;
             $variants = $changes[$i] ?? array();
             $output[] = $variants ? $this->scopeRouteUnit($base[$i], array_map($routeClass, array_keys($variants)), false) : $base[$i];
+            if ($variants) $routeBytes += strlen($output[count($output) - 1]['text']) - strlen($base[$i]['text']);
             $emitGroups(array_map(static fn ($unit): array => null === $unit ? array() : array($unit), $variants));
         }
-        return array('stylesheet' => $this->serializeRuleUnits($output), 'scoped' => $scoped, 'reasons' => $reasons);
+        return array('stylesheet' => $this->serializeRuleUnits($output), 'scoped' => $scoped, 'reasons' => $reasons, 'route_bytes' => $routeBytes);
     }
 
     /**
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index 2dde3e30e..a15b5da72 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -451,6 +451,110 @@
     echo 'Variant fixture (' . $label . ') author CSS: ' . strlen($variantText) . ' bytes in ' . count($authoredAssets($variantPlan)) . " assets\n";
 }
 
+// Bounded route scoping. One long selector list is projected three ways
+// (span, button and image pages), so in-place variants would repeat every
+// arm under route scopes on the sheet every route loads: several times what
+// one page copy costs. That sheet falls back to page copies, with a reason.
+$boundArms = array();
+for ($i = 0; $i < 60; ++$i) $boundArms[] = '.long-selector-arm-' . $i;
+$boundSheet = array('path' => 'base.css', 'kind' => 'css', 'content' => '.sample{color:#123456}' . implode(',', $boundArms) . ',button,img{outline-style:solid;letter-spacing:1px;padding:3px}');
+$boundFiles = array();
+foreach (array('<span class="sample">Probe</span>', '<span class="sample">Probe</span>', '<span class="sample">Probe</span>', '<button class="sample">Probe</button>', '<img src="p.png" alt="P">', '<button class="sample">Probe</button><img src="p.png" alt="P">') as $i => $body) {
+    $boundFiles[] = array('path' => 'bound-' . $i . '.html', 'kind' => 'html', 'content' => '<!doctype html><html><head><title>Bound</title><link rel="stylesheet" href="base.css"></head><body><main>' . $body . '</main></body></html>');
+}
+$bound = (new ArtifactCompiler())->compile(array('entrypoint' => 'bound-0.html', 'files' => array_merge($boundFiles, array($boundSheet))))->toArray();
+$boundPlan = $bound['source_reports']['wordpress_site_plan'];
+$boundFindings = array_values(array_filter($bound['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '') && str_contains($row['message'], 'largest')));
+$assert(1 === count($boundFindings) && str_contains($boundFindings[0]['message'], 'base.css'), 'over-budget route scoping emits one sheet-level fallback diagnostic', json_encode(array_column($bound['diagnostics'], 'message')));
+$boundAssets = $authoredAssets($boundPlan);
+$assert(count($boundAssets) > 1 && !str_contains(implode('', $boundAssets), 'be-route-'), 'over-budget route scoping falls back to page copies without route scopes', json_encode(array_map('strlen', $boundAssets)));
+foreach ($boundPlan['pages'] as $row) {
+    $file = array_column($boundFiles, null, 'path')[$row['source_path']];
+    $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $file['path'], 'files' => array($file, $boundSheet)))->toArray()['source_reports']['wordpress_site_plan'];
+    $boundProperties = array('color', 'outline-style', 'letter-spacing');
+    $actual = $probeStyles($boundPlan, $row, false, $boundProperties);
+    $expected = $probeStyles($alone, $alone['pages'][0], false, $boundProperties);
+    $assert(array() !== $actual && $expected === $actual, 'fallback computed-style parity ' . $row['source_path'], json_encode(array($expected, $actual)));
+}
+echo 'Bound fixture author CSS: ' . json_encode(array_map('strlen', $boundAssets)) . "\n";
+
+// Rules that differ between pages only by page-local block markers. Every
+// page has its own buttons in a `.row-cols-3` grid, so `.row-cols-3>*` and
+// `.row>*` project onto each page's own per-element markers, which the page
+// projection packs as `>:is(:where(.m1),…)`. Those differences fold into one
+// unscoped union in the shared rule: the sheet grows by marker names only,
+// never by a route-scoped copy of the rule per page.
+$markerCss = '.row{display:flex;flex-wrap:wrap}.row-cols-3>*{flex:0 0 auto;width:33.3%}.row>*{flex-shrink:0;max-width:100%}'
+    . '.btn{background-color:#123456;color:#fff;border-radius:4px;padding:8px 16px}';
+for ($i = 0; $i < 120; ++$i) $markerCss .= '.marker-tail-' . $i . '{color:#123456;margin:0;padding:2px;border-top-width:1px}';
+$markerSheet = array('path' => 'base.css', 'kind' => 'css', 'content' => $markerCss);
+$markerFile = static function (int $index, bool $gaps = false): array {
+    $buttons = '';
+    if (!$gaps || 2 !== $index % 3) for ($k = 0; $k <= $index % 3; ++$k) $buttons .= '<a class="btn" href="/m' . $index . '/' . $k . '">Buy ' . $index . '.' . $k . '</a>';
+    return array('path' => 'm' . $index . '.html', 'kind' => 'html', 'content' => '<!doctype html><html><head><title>Marker ' . $index . '</title><link rel="stylesheet" href="base.css"></head><body><main><h1>Page ' . $index . '</h1><div class="row row-cols-3">' . $buttons . '</div></main></body></html>');
+};
+$markerSite = static function (int $count, bool $gaps = false) use ($markerFile, $markerSheet, $authoredAssets): array {
+    $files = array();
+    for ($i = 0; $i < $count; ++$i) $files[] = $markerFile($i, $gaps);
+    $result = (new ArtifactCompiler())->compile(array('entrypoint' => 'm0.html', 'files' => array_merge($files, array($markerSheet))))->toArray();
+    $plan = $result['source_reports']['wordpress_site_plan'];
+    return array('plan' => $plan, 'assets' => $authoredAssets($plan), 'diagnostics' => $result['diagnostics']);
+};
+$markerSizes = array();
+foreach (array(2, 6, 10) as $count) {
+    $site = $markerSite($count);
+    $text = implode('', $site['assets']);
+    $markerSizes[$count] = strlen($text);
+    $assert(1 === count($site['assets']) && !str_contains(implode('', array_column($site['plan']['assets'], 'source_path')), '.page-'), $count . ' marker-only pages: one site-wide asset, no page copies', json_encode(array_keys($site['assets'])));
+    $assert(!str_contains($text, 'be-route-'), $count . ' marker-only pages: nothing is route scoped', $text);
+    $assert(array() === array_filter($site['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '')), $count . ' marker-only pages: no route-scope fallback');
+}
+$markerText = implode('', $site['assets']);
+$assert((bool) preg_match('/:where\((?:\.blocks-engine-control-[a-f0-9]{12}-\d+,){3,}/', $markerText), 'page-local markers fold into one recompacted :where() list', $markerText);
+// Ten pages add only their marker names: under a fifth of the two-page
+// sheet. Route-scoped copies grew it by about 2.3 KB per page.
+$assert($markerSizes[10] < 1.2 * $markerSizes[2] && $markerSizes[10] - $markerSizes[6] < 1.2 * ($markerSizes[6] - $markerSizes[2]) + 512, 'site-wide sheet bytes stay roughly constant in the page count', json_encode($markerSizes));
+$markerProbe = static function (array $plan, array $row): array {
+    $assets = array_column($plan['assets'], null, 'target_path');
+    $css = '';
+    foreach (WordPressSitePlan::documentStylesheets($plan, $row['source_path']) as $sheet) $css .= "\n" . ($assets[$sheet['target_path']]['content'] ?? '');
+    $dom = new DOMDocument();
+    @$dom->loadHTML('<html><body>' . StaticStyleParityRunner::candidateHtmlFromSerializedBlocks($row['canonical_block_markup']) . '</body></html>');
+    foreach ($row['document_metadata']['root_attributes'] ?? array() as $name => $value) $dom->documentElement->setAttribute($name, $value);
+    $cascade = new StaticCssCascade($dom, $css);
+    $styles = array();
+    foreach ($dom->getElementsByTagName('*') as $element) {
+        if (!$element instanceof DOMElement) continue;
+        $classes = ' ' . $element->getAttribute('class') . ' ';
+        if (str_contains($classes, ' wp-block-buttons ') || str_contains($classes, ' wp-block-button ') || str_contains($classes, ' wp-block-button__link ')) {
+            $styles[] = $cascade->resolve($element, array('width', 'flex-shrink', 'max-width', 'background-color', 'color'), array());
+        }
+    }
+    return $styles;
+};
+// With every third page empty, the empty pages' projection is the shared
+// one: the other pages' marker rules are page-only. Each is inert off its
+// own page, so they join the sheet as unscoped unions; only rules whose arms
+// can match without a marker carry a route scope.
+$gapSite = $markerSite(9, true);
+$gapText = implode('', $gapSite['assets']);
+$scopedMarkerRule = false;
+$transformer->visitStyleRules($gapText, static function (string $prelude) use (&$scopedMarkerRule): void {
+    if (str_contains($prelude, 'be-route-') && preg_match('/blocks-engine-control-[a-f0-9]{12}-\d+/', $prelude)) $scopedMarkerRule = true;
+});
+$assert(1 === count($gapSite['assets']) && !$scopedMarkerRule, 'page-only marker rules fold into unscoped unions', $gapText);
+foreach (array(array($site, false), array($gapSite, true)) as [$checked, $gaps]) {
+    foreach ($checked['plan']['pages'] as $row) {
+        $index = (int) substr($row['source_path'], 1);
+        $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $row['source_path'], 'files' => array($markerFile($index, $gaps), $markerSheet)))->toArray()['source_reports']['wordpress_site_plan'];
+        $actual = $markerProbe($checked['plan'], $row);
+        $expected = $markerProbe($alone, $alone['pages'][0]);
+        $hasGrid = !$gaps || 2 !== $index % 3;
+        $assert($expected === $actual && $hasGrid === in_array('33.3%', array_column($actual, 'width'), true), 'marker-only computed-style parity ' . $row['source_path'] . ($gaps ? ' (with empty pages)' : ''), json_encode(array($expected, $actual)));
+    }
+}
+echo 'Marker-only fixture author CSS: ' . json_encode($markerSizes) . " bytes for 2/6/10 pages\n";
+
 // Identical shared units are one fold match, rewritten at every occurrence.
 $marker = 'blocks-engine-control-0123456789ab-0';
 $unit = '.group>.btn{z-index:1}';
diff --git a/php-transformer/tests/unit/css-stylesheet-transformer.php b/php-transformer/tests/unit/css-stylesheet-transformer.php
index 101c79af6..15a5267cd 100644
--- a/php-transformer/tests/unit/css-stylesheet-transformer.php
+++ b/php-transformer/tests/unit/css-stylesheet-transformer.php
@@ -213,6 +213,20 @@ static function (string $name, string $body) use (&$visitedKeyframes): void {
 $assert($namespaceCss === \Automattic\BlocksEngine\PhpTransformer\Css\CssSelectorListCompactor::stylesheet($namespaceCss), 'namespace-sensitive sheets remain unchanged');
 $splitRules = '.catalog .one{color:red}.catalog .other{color:blue}.catalog .two{color:red}';
 $assert($splitRules === \Automattic\BlocksEngine\PhpTransformer\Css\CssSelectorListCompactor::stylesheet($splitRules), 'identical declarations never merge across an intervening rule');
+// Page-local marker lists expand back into arms for cross-page alignment and
+// merge into one zero-weight :where() list for the site-wide sheet.
+$compactor = \Automattic\BlocksEngine\PhpTransformer\Css\CssSelectorListCompactor::class;
+$m1 = 'blocks-engine-control-4041d29655d8-0';
+$m2 = 'blocks-engine-control-f9b2c417568b-0';
+$packed = $compact('.row-cols-6>:where(.' . $m1 . '),.row-cols-6>:where(.' . $m2 . ')');
+$assert('.row-cols-6>:is(:where(.' . $m1 . '),:where(.' . $m2 . '))' === $packed, 'per-page projection packs marker arms');
+$assert('.row-cols-6>:where(.' . $m1 . '),.row-cols-6>:where(.' . $m2 . ')' === $compactor::expand($packed), 'packed marker arms expand back into one arm per marker');
+$merged = $compactor::mergeMarkerLists('.row-cols-6>:where(.' . $m1 . '):not(.be-c-0),#x,.row-cols-6>:where(.' . $m2 . '):not(.be-c-0)');
+$assert('.row-cols-6>:where(.' . $m1 . ',.' . $m2 . '):not(.be-c-0),#x' === $merged, 'arms differing by one marker merge into a :where() list');
+$assert('.row-cols-6>:where(.' . $m1 . '):not(.be-c-0),.row-cols-6>:where(.' . $m2 . '):not(.be-c-0),#x' === $compactor::expand($merged), 'merged marker lists expand back into arms');
+$assert('.a :is(.one,.two)' === $compactor::expand('.a :is(.one,.two)') && ':not(:where(.' . $m1 . ',.' . $m2 . ')) a,b' === $compactor::mergeMarkerLists(':not(:where(.' . $m1 . ',.' . $m2 . ')) a,b'), 'non-marker lists and negated marker lists stay as authored');
+$unequalMarkers = '.a :is(:where(.' . $m1 . '),#b)';
+$assert($unequalMarkers === $compactor::expand($unequalMarkers), 'unequal-weight alternatives are never expanded');
 $before = '.catalog .one,.catalog .two,.catalog .three';
 $after = $compact($before);
 $matcher = \Automattic\BlocksEngine\PhpTransformer\Css\CssSelectorMatcher::class;

From ff5af83c71dfc5137d123006b44bdc2404f28378 Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Mon, 26 Oct 2026 22:16:00 -0400
Subject: [PATCH 04/10] Reconcile route stylesheet variants per selector arm

---
 .../src/ArtifactCompiler/ArtifactCompiler.php |  15 +-
 .../src/Css/CssSelectorListCompactor.php      |  11 +
 .../src/Css/CssStylesheetTransformer.php      | 251 +++++-------------
 .../shared-author-stylesheet-scaling.php      | 109 ++++++--
 4 files changed, 176 insertions(+), 210 deletions(-)

diff --git a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
index 1fe648db9..62c5422f9 100644
--- a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
+++ b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
@@ -2650,27 +2650,24 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             }
             unset($contents);
             $sharedStylesheets = array($transformer->concatenateWithoutRedundantRules($sharedStylesheets));
-            $folded = $transformer->foldPositiveMarkerArms($sharedStylesheets, $pageProjections, $pageMarkup);
-            $sharedStylesheets = $folded['shared'];
             $fallbackSharedStylesheets = $sharedStylesheets;
-            foreach ($pages as $owner => $_) $pages[$owner] = $folded['pages'][$owner] ?? $pages[$owner];
-            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $folded['pages'], $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup);
-            // Folded rules carry expanded marker arms; recompact the serialized
-            // sheet, merging marker arms into `:where()` lists across routes.
-            $recompact = static fn (string $css): string => count($folded['pages']) > 1 ? CssSelectorListCompactor::stylesheet($css, true) : $css;
+            foreach ($pages as $owner => $_) $pages[$owner] = $pageProjections[$owner] ?? $pages[$owner];
+            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $pageProjections, $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup);
+            // Recompact only at serialization, merging compatible marker arms.
+            $recompact = static fn (string $css): string => count($pageProjections) > 1 ? CssSelectorListCompactor::stylesheet($css, true) : $css;
             // Bound what route scoping adds to the sheet every route loads by
             // what the largest page copy would have added to one page. Both
             // are measured before compaction.
             $largestCopy = 0;
             if (0 < $routed['route_bytes']) {
                 foreach ($routed['scoped'] as $owner) {
-                    $largestCopy = max($largestCopy, strlen($transformer->rulesAbsentFrom($folded['pages'][$owner] ?? array(), $fallbackSharedStylesheets, true)));
+                    $largestCopy = max($largestCopy, strlen($transformer->rulesAbsentFrom($pageProjections[$owner] ?? array(), $fallbackSharedStylesheets, true)));
                 }
             }
             $routeBudget = max(self::ROUTE_SCOPE_BYTES_FACTOR * $largestCopy, self::ROUTE_SCOPE_BYTES_FLOOR);
             if (array() !== $routed['scoped'] && $routed['route_bytes'] > $routeBudget) {
                 $projectionDiagnostics[] = $this->diagnostic('author_stylesheet_route_scope_fallback', 'warning', sprintf('%s: route-scoped variants add %d bytes to every route, over %d (%dx the largest %d-byte page copy); emitting page copies', $path, $routed['route_bytes'], $routeBudget, self::ROUTE_SCOPE_BYTES_FACTOR, $largestCopy));
-                foreach ($routed['scoped'] as $owner) $pages[$owner] ??= $folded['pages'][$owner] ?? array();
+                foreach ($routed['scoped'] as $owner) $pages[$owner] ??= $pageProjections[$owner] ?? array();
                 $routed['stylesheet'] = implode("\n", $fallbackSharedStylesheets);
             } else {
                 foreach ($routed['scoped'] as $owner) unset($pages[$owner]);
diff --git a/php-transformer/src/Css/CssSelectorListCompactor.php b/php-transformer/src/Css/CssSelectorListCompactor.php
index 242f815e8..8b03b78b5 100644
--- a/php-transformer/src/Css/CssSelectorListCompactor.php
+++ b/php-transformer/src/Css/CssSelectorListCompactor.php
@@ -139,6 +139,17 @@ public static function expand(string $prelude): string
     /** @return list<string>|null */
     private static function expandArm(string $arm): ?array
     {
+        // Richtext carriers use style attributes rather than marker classes.
+        $identity = 'blocks-engine-richtext-[a-f0-9]{12}-[0-9]+';
+        $richtext = '(?:mark\\[style\\*="--blocks-engine-richtext-marker:' . $identity . ';"\\]|mark\\[style\\$="--blocks-engine-richtext-marker:' . $identity . '"\\]|span\\[data-blocks-engine-richtext-marker="' . $identity . '"\\])';
+        if ( ! str_contains($arm, '\\') && preg_match_all('/:where\\(((?:' . $richtext . ',)+' . $richtext . ')\\)/', $arm, $lists, PREG_OFFSET_CAPTURE) ) {
+            foreach ( $lists[0] as $index => [$text, $offset] ) {
+                $before = substr($arm, 0, $offset);
+                if ( substr_count($before, '(') !== substr_count($before, ')') ) continue;
+                $suffix = substr($arm, $offset + strlen($text));
+                return array_map(static fn(string $marker): string => $before . ':where(' . $marker . ')' . $suffix, explode(',', $lists[1][$index][0]));
+            }
+        }
         if ( false === strpbrk($arm, '\\"\'') && preg_match_all(self::MARKER_WHERE, $arm, $matches, PREG_OFFSET_CAPTURE) ) {
             foreach ( $matches[0] as $index => [$text, $offset] ) {
                 $before = substr($arm, 0, $offset);
diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 166d32535..088168984 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -220,113 +220,6 @@ public function rulesAbsentFrom(array $stylesheets, array $present, bool $preser
         return $this->serializeRuleUnits($kept);
     }
 
-    /**
-     * Fold page-only positive identity-marker arms into an unambiguous shared
-     * rule, at its original cascade position. A carrier on any consuming page
-     * must have projected the very same arm on that rule. Missing page evidence
-     * is deliberately unsafe. Return equivalent page rules using the union so
-     * rulesAbsentFrom() can remove them without moving a cascade suffix.
-     *
-     * Reconciliation runs on uncompacted arms: per-page projections pack
-     * marker arms into `P:is(:where(.m1),…)S`, so every compared rule is
-     * expanded back to one arm per marker (rules left as they are keep their
-     * text). A marker arm is any arm with a top-level positive
-     * `:where(.marker)`, wherever it sits in the selector: it cannot match an
-     * element that does not carry the marker. Folded rules
-     * are uncompacted; the caller recompacts the final serialized sheet.
-     *
-     * @param list<string> $shared
-     * @param array<string, list<string>> $pages All consuming page projections.
-     * @param array<string, string> $markup Canonical markup by page owner.
-     * @return array{shared: list<string>, pages: array<string, list<string>>}
-     */
-    public function foldPositiveMarkerArms(array $shared, array $pages, array $markup): array
-    {
-        $unchanged = array('shared' => $shared, 'pages' => $pages);
-        foreach (array_merge($shared, ...array_values($pages)) as $css) {
-            if (!$this->isWellFormedStylesheet($css)) return $unchanged;
-        }
-        $carriers = self::markerCarriers($markup);
-        $markerOf = static fn (string $arm): ?string => self::positiveMarker($arm);
-        $describe = static function (array $unit) use ($markerOf): ?array {
-            $brace = strpos($unit['text'], '{');
-            if (!$unit['dedupable'] || false === $brace) return null;
-            $prelude = trim(substr($unit['text'], 0, $brace));
-            if (str_starts_with($prelude, '@')) return null;
-            $arms = self::splitSelectorList(CssSelectorListCompactor::expand($prelude));
-            if (null === $arms) return null;
-            $arms = array_map('trim', $arms);
-            $constant = array_values(array_filter($arms, static fn (string $arm): bool => null === $markerOf($arm)));
-            sort($constant);
-            return array('key' => serialize(array($unit['context'], substr($unit['text'], $brace), $constant)), 'arms' => $arms, 'brace' => $brace);
-        };
-        $sharedUnits = array();
-        foreach ($shared as $css) $this->collectRuleUnits($css, array(), $sharedUnits);
-        $index = array();
-        foreach ($sharedUnits as $i => $unit) {
-            $description = $describe($unit);
-            if (null !== $description) $index[$description['key']][] = $i;
-        }
-        $pageUnits = array();
-        $pageIndex = array();
-        foreach ($pages as $owner => $sheets) {
-            $pageUnits[$owner] = array();
-            foreach ($sheets as $css) $this->collectRuleUnits($css, array(), $pageUnits[$owner]);
-            foreach ($pageUnits[$owner] as $i => $unit) {
-                $description = $describe($unit);
-                if (null !== $description) $pageIndex[$owner][$description['key']][] = $i;
-            }
-        }
-        foreach ($index as $key => $matches) {
-            if (1 !== count(array_unique(array_map(static fn (int $i): string => $sharedUnits[$i]['text'], $matches)))) continue;
-            $i = $matches[0];
-            $base = $describe($sharedUnits[$i]);
-            $union = $base['arms'];
-            $candidates = array();
-            foreach ($pageIndex as $owner => $rules) {
-                $pageMatches = $rules[$key] ?? array();
-                if (!$pageMatches || 1 !== count(array_unique(array_map(static fn (int $j): string => $pageUnits[$owner][$j]['text'], $pageMatches)))) continue;
-                $description = $describe($pageUnits[$owner][$pageMatches[0]]);
-                foreach (array_diff($description['arms'], $union) as $arm) {
-                    $marker = $markerOf($arm);
-                    if (null !== $marker) $candidates[$arm] = $marker;
-                }
-            }
-            foreach ($candidates as $arm => $marker) {
-                $safe = true;
-                if (array_diff_key($pages, $markup)) $safe = false;
-                foreach ($carriers[$marker] ?? array() as $owner => $_) {
-                    $rules = $pageIndex[$owner][$key] ?? array();
-                    if (!$rules) $safe = false;
-                    foreach ($rules as $j) if (!in_array($arm, $describe($pageUnits[$owner][$j])['arms'], true)) $safe = false;
-                    if (!$safe) break;
-                }
-                if ($safe) $union[] = $arm;
-            }
-            if ($union !== $base['arms']) {
-                foreach ($matches as $match) $sharedUnits[$match]['text'] = implode(',', $union) . substr($sharedUnits[$match]['text'], $base['brace']);
-            }
-            // A page rule becomes the shared text when the shared rule only
-            // adds arms whose marker the page does not carry, whatever the
-            // packing of either side.
-            foreach ($pageIndex as $owner => $rules) {
-                if (!isset($markup[$owner])) continue;
-                foreach ($rules[$key] ?? array() as $j) {
-                    if ($pageUnits[$owner][$j]['text'] === $sharedUnits[$i]['text']) continue;
-                    $description = $describe($pageUnits[$owner][$j]);
-                    if (array() !== array_diff($description['arms'], $union)) continue;
-                    foreach (array_diff($union, $description['arms']) as $arm) {
-                        if (isset($carriers[(string) $markerOf($arm)][$owner])) continue 2;
-                    }
-                    $pageUnits[$owner][$j]['text'] = $sharedUnits[$i]['text'];
-                }
-            }
-        }
-        $result = array('shared' => array($this->serializeRuleUnits($sharedUnits)), 'pages' => array());
-        foreach ($pageUnits as $owner => $units) $result['pages'][$owner] = array($this->serializeRuleUnits($units));
-        return $result;
-    }
-
     /**
      * Owners whose markup carries each page-local marker.
      *
@@ -346,16 +239,19 @@ private static function markerCarriers(array $markup): array
     /**
      * The page-local marker an arm requires: a literal `:where(.marker)` at
      * selector top level, outside every `:not()`/`:is()`/`:has()` argument.
-     * Comments, escapes and strings are refused, so a marker negation, a
-     * selector-list alternative or quoted text never counts as a requirement.
+     * Also accepts the literal richtext-marker style attribute form. Nested
+     * alternatives, negations, comments and escapes do not prove a carrier.
      */
     private static function positiveMarker(string $arm): ?string
     {
-        if (str_contains($arm, '/*') || strpbrk($arm, '\\"\'') !== false) return null;
-        if (!preg_match_all('/:where\(\.(blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+)\)/', $arm, $matches, PREG_OFFSET_CAPTURE)) return null;
+        if (str_contains($arm, '/*') || str_contains($arm, '\\')) return null;
+        $marker = '(blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+)';
+        $pattern = '/:where\((?:\.' . $marker . '|mark\[style\*="--blocks-engine-richtext-marker:' . $marker . ';"\]|mark\[style\$="--blocks-engine-richtext-marker:' . $marker . '"\]|span\[data-blocks-engine-richtext-marker="' . $marker . '"\])\)/';
+        if (!preg_match_all($pattern, $arm, $matches, PREG_OFFSET_CAPTURE)) return null;
         foreach ($matches[0] as $index => [, $offset]) {
             $before = substr($arm, 0, $offset);
-            if (substr_count($before, '(') === substr_count($before, ')') && substr_count($before, '[') === substr_count($before, ']')) return $matches[1][$index][0];
+            if (0 !== self::parenDepth($before) || substr_count($before, '[') !== substr_count($before, ']')) continue;
+            foreach (array_slice($matches, 1) as $captures) if ('' !== ($captures[$index][0] ?? '')) return $captures[$index][0];
         }
         return null;
     }
@@ -371,39 +267,21 @@ private static function positiveMarker(string $arm): ?string
      *
      * @param list<string> $shared
      * @param array<string,list<string>> $pages
-     * Rules whose every arm requires a page-local marker (see
-     * positiveMarker()) need no route scope when $markup proves where the
-     * markers live: a route that drops such a rule and carries none of its
-     * markers loses nothing, and a route-only insertion whose markers only
-     * that route carries is emitted unscoped, merged by arm union with the
-     * other routes' insertions of the same rule at the same position.
+     * Aligned rules are expanded into arms, each with its own route set.
+     * Marker arms inert outside that set stay unscoped; all others use the
+     * shorter positive or complementary negative scope. Only validated lists
+     * are split, preserving invalid-selector isolation and source order.
      *
      * @param array<string,string> $routeIdentities Route identities by compilation owner.
      * @param array<string,string> $markup Canonical markup by compilation owner.
-     * `route_bytes` counts what route scoping adds: positively scoped rules
-     * plus the growth of negatively scoped shared rules, before compaction.
+     * `route_bytes` counts scope and split-rule overhead beyond the unscoped
+     * arm union, plus scoped insertions, before final compaction.
      *
      * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>, route_bytes:int}
      */
     public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array()): array
     {
         $carriers = self::markerCarriers($markup);
-        // Owners that can carry every marker the unit's arms require, or null
-        // when some arm can match without a page-local marker.
-        $markerOwners = function (array $unit) use ($carriers): ?array {
-            [, $rule] = self::splitLeadingComments($unit['text']);
-            $brace = $this->nextRuleBoundary($rule, 0);
-            if (null === $brace || '{' !== $rule[$brace] || $this->isAtRule($rule) || !$unit['dedupable']) return null;
-            $arms = self::splitSelectorList(CssSelectorListCompactor::expand(substr($rule, 0, $brace)));
-            if (null === $arms) return null;
-            $owners = array();
-            foreach ($arms as $arm) {
-                $marker = self::positiveMarker(trim($arm));
-                if (null === $marker) return null;
-                $owners += $carriers[$marker] ?? array();
-            }
-            return $owners;
-        };
         $base = array();
         foreach ($shared as $css) $this->collectRuleUnits($css, array(), $base);
         $normal = static fn (array $unit): string => trim(CssSyntaxScanner::withoutComments($unit['text']));
@@ -450,13 +328,6 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                 $cursor = $match + 1;
             }
             while ($cursor < count($base)) $localChanges[$cursor++] = null;
-            $known = isset($markup[$owner]);
-            foreach ($localChanges as $i => $variant) {
-                if (null !== $variant || !$known) continue;
-                $owners = $markerOwners($base[$i]);
-                // Dropping a rule this route cannot match changes nothing.
-                if (null !== $owners && !isset($owners[$owner])) unset($localChanges[$i]);
-            }
             // A definition the route drops or adds stays site-wide, unless the
             // route would then see a different body under the same name.
             $pageDefined = $defined;
@@ -480,12 +351,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                     $this->scopeRouteUnit($unit, array('be-route-probe'), true, $reason);
                 }
             }
-            foreach ($localInsertions as $i => $list) foreach ($list as $ordinal => $unit) {
-                $owners = $known ? $markerOwners($unit) : null;
-                if (null !== $owners && array((string) $owner) === array_map('strval', array_keys($owners))) {
-                    $localInsertions[$i][$ordinal]['own_markers'] = true;
-                    continue;
-                }
+            foreach ($localInsertions as $list) foreach ($list as $unit) {
                 $this->scopeRouteUnit($unit, array('be-route-probe'), true, $reason);
             }
             if (null !== $reason) { $reasons[$owner] = $reason; continue; }
@@ -502,48 +368,67 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         $output = array();
         $routeClass = static fn ($owner): string => \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($routeIdentities[$owner] ?? (string) $owner);
         $routeBytes = 0;
-        $emitGroups = function (array $groups) use (&$output, &$routeBytes, $routeClass): void {
-            $texts = array();
-            foreach ($groups as $owner => $units) foreach ($units as $ordinal => $unit) {
-                if (!empty($unit['own_markers'])) {
-                    // Inert off its route: union the arms, no route scope.
-                    unset($unit['own_markers']);
-                    [$lead, $rule] = self::splitLeadingComments($unit['text']);
-                    $brace = (int) $this->nextRuleBoundary($rule, 0);
-                    $groupKey = serialize(array($ordinal, $unit['context'], substr($rule, $brace), 'union'));
-                    $texts[$groupKey]['ordinal'] = $ordinal;
-                    $texts[$groupKey]['unit'] ??= $unit;
-                    $texts[$groupKey]['lead'] ??= $lead;
-                    $texts[$groupKey]['body'] = substr($rule, $brace);
-                    foreach ((array) self::splitSelectorList(CssSelectorListCompactor::expand(substr($rule, 0, $brace))) as $arm) $texts[$groupKey]['arms'][trim($arm)] = true;
-                    continue;
+        $allRoutes = array_fill_keys(array_keys($pages), true);
+        $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $carriers, $markup): void {
+            [$lead, $rule] = self::splitLeadingComments($prototype['text']);
+            $brace = (int) $this->nextRuleBoundary($rule, 0);
+            $body = substr($rule, $brace);
+            $members = array();
+            foreach ($units as $owner => $unit) {
+                if (null === $unit) continue;
+                [, $text] = self::splitLeadingComments($unit['text']);
+                $start = (int) $this->nextRuleBoundary($text, 0);
+                foreach ((array) self::splitSelectorList(CssSelectorListCompactor::expand(CssSyntaxScanner::withoutComments(substr($text, 0, $start)))) as $arm) {
+                    $members[trim($arm)][$owner] = true;
                 }
-                $groupKey = serialize(array($ordinal, $unit));
-                $texts[$groupKey]['ordinal'] = $ordinal;
-                $texts[$groupKey]['unit'] = $unit;
-                $texts[$groupKey]['routes'][] = $routeClass($owner);
             }
-            // Each route has one unit per ordinal; ordinal order keeps every
-            // route's own insertions in its source order.
-            uasort($texts, static fn (array $a, array $b): int => $a['ordinal'] <=> $b['ordinal']);
-            foreach ($texts as $group) {
-                if (isset($group['arms'])) {
-                    $group['unit']['text'] = $group['lead'] . implode(',', array_keys($group['arms'])) . $group['body'];
-                    $output[] = $group['unit'];
-                    continue;
+            $groups = array();
+            foreach ($members as $arm => $routes) {
+                $outside = array_diff_key($allRoutes, $routes);
+                $marker = self::positiveMarker($arm);
+                $inert = !$outside || (null !== $marker && !array_diff_key($outside, $markup) && !array_diff_key($carriers[$marker] ?? array(), $routes));
+                $scope = '';
+                if (!$inert) {
+                    $positive = ':where(:root:is(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($routes))) . '))';
+                    $negative = ':where(:root:not(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($outside))) . '))';
+                    $scope = strlen($positive) <= strlen($negative) ? $positive : $negative;
                 }
-                $output[] = $this->scopeRouteUnit($group['unit'], $group['routes'], true);
-                $routeBytes += strlen(end($output)['text']);
+                $groups[$scope][] = $arm;
             }
+            $emittedBytes = 0;
+            $unionBytes = strlen($lead . implode(',', array_keys($members)) . $body);
+            foreach ($groups as $scope => $arms) {
+                $unit = $prototype;
+                if ('' !== $scope) {
+                    $reason = null;
+                    $arms = self::scopeRouteArms($arms, $scope, $reason);
+                }
+                $unit['text'] = $lead . implode(',', $arms) . $body;
+                $output[] = $unit;
+                $emittedBytes += strlen($unit['text']);
+                // A scoped insertion is entirely new shared-download cost,
+                // not just the prefix added to its unscoped selector list.
+                if ($insertion && '' !== $scope) $routeBytes += strlen(implode(',', $groups[$scope]) . $body);
+                $lead = '';
+            }
+            $routeBytes += max(0, $emittedBytes - $unionBytes);
         };
         for ($i = 0; $i <= count($base); ++$i) {
             foreach ($siteInsertions[$i] ?? array() as $unit) $output[] = $unit;
-            $emitGroups($insertions[$i] ?? array());
+            $groups = array();
+            foreach ($insertions[$i] ?? array() as $owner => $units) foreach ($units as $ordinal => $unit) {
+                $groupKey = serialize(array($ordinal, $key($unit)));
+                $groups[$groupKey]['ordinal'] = $ordinal;
+                $groups[$groupKey]['unit'] = $unit;
+                $groups[$groupKey]['routes'][$owner] = $unit;
+            }
+            uasort($groups, static fn (array $a, array $b): int => $a['ordinal'] <=> $b['ordinal']);
+            foreach ($groups as $group) $emitArms($group['routes'], $group['unit'], true);
             if ($i === count($base)) break;
-            $variants = $changes[$i] ?? array();
-            $output[] = $variants ? $this->scopeRouteUnit($base[$i], array_map($routeClass, array_keys($variants)), false) : $base[$i];
-            if ($variants) $routeBytes += strlen($output[count($output) - 1]['text']) - strlen($base[$i]['text']);
-            $emitGroups(array_map(static fn ($unit): array => null === $unit ? array() : array($unit), $variants));
+            if (empty($changes[$i])) { $output[] = $base[$i]; continue; }
+            $units = array();
+            foreach ($allRoutes as $owner => $_) $units[$owner] = array_key_exists($owner, $changes[$i]) ? $changes[$i][$owner] : $base[$i];
+            $emitArms($units, $base[$i]);
         }
         return array('stylesheet' => $this->serializeRuleUnits($output), 'scoped' => $scoped, 'reasons' => $reasons, 'route_bytes' => $routeBytes);
     }
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index a15b5da72..a836c0609 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -275,7 +275,7 @@
 }
 echo 'Context fixture author CSS: ' . $contextBytes . " bytes\n";
 
-// Compare the old enqueue cascade with the folded one directly, including
+// Compare the old enqueue cascade with arm reconciliation directly, including
 // every overlapping tail rule. Markers absent from another page are inert.
 $transformer = new Automattic\BlocksEngine\PhpTransformer\Css\CssStylesheetTransformer();
 $shared = '.group>.btn{z-index:1}';
@@ -287,27 +287,25 @@
     $projections[$i] = array(str_replace('.group>.btn', '.group>.btn,:where(.' . $marker . ')', $shared));
     $markup[$i] = '<p class="' . $marker . ' tail-159">Probe</p>';
 }
-$folded = $transformer->foldPositiveMarkerArms(array($shared), $projections, $markup);
+$routed = $transformer->scopeRouteVariants(array($shared), $projections, array(), null, $markup);
 foreach ($projections as $owner => $sheets) {
     $before = $shared . $transformer->rulesAbsentFrom($sheets, array($shared), true);
-    $after = implode('', $folded['shared']) . $transformer->rulesAbsentFrom($folded['pages'][$owner], $folded['shared'], true);
+    $after = $routed['stylesheet'];
     $document = new DOMDocument();
     @$document->loadHTML($markup[$owner]);
     $element = $document->getElementsByTagName('p')->item(0);
     $oldStyle = (new StaticCssCascade($document, $before))->resolve($element, array('z-index'), array());
     $newStyle = (new StaticCssCascade($document, $after))->resolve($element, array('z-index'), array());
-    $assert($oldStyle === $newStyle && '2' === (string) ($newStyle['z-index'] ?? ''), 'folding preserves the original enqueue cascade on page ' . $owner);
+    $assert($oldStyle === $newStyle && '2' === (string) ($newStyle['z-index'] ?? ''), 'arm reconciliation preserves the original enqueue cascade on page ' . $owner);
 }
 
 // A reused identity is not proof of a shared match: both pages carry the
-// marker, but only one projected the arm. Keep it out of the marker fold.
+// marker, but only one projected the arm. Scope that arm alone.
 $marker = 'blocks-engine-control-0123456789ab-0';
 $base = '.btn-group>.btn{z-index:1}.tail{z-index:2}';
 $extra = '.btn-group>.btn,:where(.' . $marker . ')>.wp-block-button__link{z-index:1}.tail{z-index:2}';
-$folded = $transformer->foldPositiveMarkerArms(array($base), array('a' => array($extra), 'b' => array($base)), array('a' => '<div class="' . $marker . '"></div>', 'b' => '<div class="' . $marker . '"></div>'));
-$assert(!str_contains(implode('', $folded['shared']), $marker), 'a marker carried by a non-projecting page is not folded');
-$routed = $transformer->scopeRouteVariants($folded['shared'], $folded['pages']);
-$assert(str_contains($routed['stylesheet'], $marker) && 2 === count($routed['scoped']), 'an arm unsafe for marker folding can still be route scoped in place');
+$routed = $transformer->scopeRouteVariants(array($base), array('a' => array($extra), 'b' => array($base)), array(), null, array('a' => '<div class="' . $marker . '"></div>', 'b' => '<div class="' . $marker . '"></div>'));
+$assert(str_contains($routed['stylesheet'], $marker) && 2 === count($routed['scoped']), 'a shared marker arm is route scoped in place');
 
 // Real constant-arm variants, a dropped control override, and page-only
 // image bridge rules. Identical span routes must share one positive rule.
@@ -451,10 +449,9 @@
     echo 'Variant fixture (' . $label . ') author CSS: ' . strlen($variantText) . ' bytes in ' . count($authoredAssets($variantPlan)) . " assets\n";
 }
 
-// Bounded route scoping. One long selector list is projected three ways
-// (span, button and image pages), so in-place variants would repeat every
-// arm under route scopes on the sheet every route loads: several times what
-// one page copy costs. That sheet falls back to page copies, with a reason.
+// A long selector list projected three ways used to exhaust the byte bound
+// by copying every arm into whole-rule variants. Only differing arms now
+// carry scope overhead, so this fixture stays below the safety budget.
 $boundArms = array();
 for ($i = 0; $i < 60; ++$i) $boundArms[] = '.long-selector-arm-' . $i;
 $boundSheet = array('path' => 'base.css', 'kind' => 'css', 'content' => '.sample{color:#123456}' . implode(',', $boundArms) . ',button,img{outline-style:solid;letter-spacing:1px;padding:3px}');
@@ -465,9 +462,9 @@
 $bound = (new ArtifactCompiler())->compile(array('entrypoint' => 'bound-0.html', 'files' => array_merge($boundFiles, array($boundSheet))))->toArray();
 $boundPlan = $bound['source_reports']['wordpress_site_plan'];
 $boundFindings = array_values(array_filter($bound['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '') && str_contains($row['message'], 'largest')));
-$assert(1 === count($boundFindings) && str_contains($boundFindings[0]['message'], 'base.css'), 'over-budget route scoping emits one sheet-level fallback diagnostic', json_encode(array_column($bound['diagnostics'], 'message')));
+$assert(array() === $boundFindings, 'arm-level scoping avoids the former whole-rule byte-budget fallback', json_encode(array_column($bound['diagnostics'], 'message')));
 $boundAssets = $authoredAssets($boundPlan);
-$assert(count($boundAssets) > 1 && !str_contains(implode('', $boundAssets), 'be-route-'), 'over-budget route scoping falls back to page copies without route scopes', json_encode(array_map('strlen', $boundAssets)));
+$assert(1 === count($boundAssets) && !str_contains(implode('', array_keys($boundAssets)), '.page-'), 'long unchanged arm lists stay in one site-wide asset', json_encode(array_map('strlen', $boundAssets)));
 foreach ($boundPlan['pages'] as $row) {
     $file = array_column($boundFiles, null, 'path')[$row['source_path']];
     $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $file['path'], 'files' => array($file, $boundSheet)))->toArray()['source_reports']['wordpress_site_plan'];
@@ -478,6 +475,28 @@
 }
 echo 'Bound fixture author CSS: ' . json_encode(array_map('strlen', $boundAssets)) . "\n";
 
+// The safety net still trips when route-list overhead itself dominates the
+// largest copy, even though no whole selector lists are duplicated anymore.
+$budgetFiles = array();
+for ($i = 0; $i < 24; ++$i) {
+    $file = $boundFiles[$i % count($boundFiles)];
+    $file['path'] = 'budget-' . $i . '.html';
+    $budgetFiles[] = $file;
+}
+$budgetCss = '.sample{color:#123456}';
+for ($i = 0; $i < 80; ++$i) $budgetCss .= 'button,img{outline-style:solid;letter-spacing:' . $i . 'px;padding:3px}';
+$budgetSheet = array('path' => 'base.css', 'kind' => 'css', 'content' => $budgetCss);
+$budgetResult = (new ArtifactCompiler())->compile(array('entrypoint' => 'budget-0.html', 'files' => array_merge($budgetFiles, array($budgetSheet))))->toArray();
+$budgetFindings = array_filter($budgetResult['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '') && str_contains($row['message'], 'largest'));
+$budgetPlan = $budgetResult['source_reports']['wordpress_site_plan'];
+$assert(1 === count($budgetFindings) && str_contains(implode('', array_column($budgetPlan['assets'], 'source_path')), '.page-'), 'excessive route-list overhead retains the sheet-level byte-budget fallback');
+foreach (array_slice($budgetPlan['pages'], 0, 2) as $row) {
+    $file = array_column($budgetFiles, null, 'path')[$row['source_path']];
+    $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $file['path'], 'files' => array($file, $budgetSheet)))->toArray()['source_reports']['wordpress_site_plan'];
+    $properties = array('color', 'outline-style', 'letter-spacing');
+    $assert($probeStyles($alone, $alone['pages'][0], false, $properties) === $probeStyles($budgetPlan, $row, false, $properties), 'byte-budget fallback computed-style parity ' . $row['source_path']);
+}
+
 // Rules that differ between pages only by page-local block markers. Every
 // page has its own buttons in a `.row-cols-3` grid, so `.row-cols-3>*` and
 // `.row>*` project onto each page's own per-element markers, which the page
@@ -555,12 +574,66 @@
 }
 echo 'Marker-only fixture author CSS: ' . json_encode($markerSizes) . " bytes for 2/6/10 pages\n";
 
-// Identical shared units are one fold match, rewritten at every occurrence.
+// Mixed real-site rule shapes: hundreds of safe marker arms must not inherit
+// a shared-chrome arm's scope. Richtext swaps and a dropped plain rule use
+// the same per-arm membership path, including complementary scopes.
+$armSizes = array();
+foreach (array(2, 10, 30) as $count) {
+    $chrome = 'blocks-engine-control-aaaaaaaaaaaa-0';
+    $richA = 'blocks-engine-richtext-bbbbbbbbbbbb-0';
+    $richB = 'blocks-engine-richtext-cccccccccccc-0';
+    $richArm = static fn (string $marker): string => ':where(mark[style*="--blocks-engine-richtext-marker:' . $marker . ';"])';
+    $safeArms = array('.plain');
+    for ($j = 0; $j < 200; ++$j) $safeArms[] = ':where(.blocks-engine-control-' . substr(hash('sha256', 'safe-' . $j), 0, 12) . '-0)';
+    $common = implode(',', $safeArms);
+    $liveSafe = 'blocks-engine-control-' . substr(hash('sha256', 'safe-0'), 0, 12) . '-0';
+    $base = '@media(min-width:1px){' . $common . ',:where(.' . $chrome . '){color:red}' . $richArm($richA) . '{background-color:blue}.drop{padding-top:7px}}';
+    $pages = $markup = array();
+    for ($j = 0; $j < $count; ++$j) {
+        $pages['arm-' . $j] = array('@media(min-width:1px){' . $common . (0 === $j ? ',:where(.' . $chrome . ')' : '') . '{color:red}' . $richArm(0 === $j ? $richA : $richB) . '{background-color:blue}' . (1 === $j ? '' : '.drop{padding-top:7px}') . '}');
+        $markup['arm-' . $j] = '<p class="' . $chrome . ' drop ' . $liveSafe . '">Chrome</p><mark style="--blocks-engine-richtext-marker:' . $richA . ';">A</mark><mark style="--blocks-engine-richtext-marker:' . $richB . ';">B</mark>';
+    }
+    $result = $transformer->scopeRouteVariants(array($base), $pages, array(), null, $markup);
+    $text = Automattic\BlocksEngine\PhpTransformer\Css\CssSelectorListCompactor::stylesheet($result['stylesheet'], true);
+    $armSizes[$count] = strlen($text);
+    $assert(count($result['scoped']) === $count && array() === $result['reasons'], $count . ' mixed-arm routes reconcile without page-copy fallback');
+    $assert(1 === substr_count($text, 'blocks-engine-control-' . substr(hash('sha256', 'safe-0'), 0, 12)), 'one shared-chrome arm never duplicates its safe sibling list');
+    foreach ($pages as $owner => $sheets) {
+        $dom = new DOMDocument();
+        @$dom->loadHTML('<html><body>' . $markup[$owner] . '</body></html>');
+        $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+        $expected = new StaticCssCascade($dom, implode('', $sheets));
+        $actual = new StaticCssCascade($dom, $text);
+        foreach (array('p', 'mark') as $tag) foreach ($dom->getElementsByTagName($tag) as $element) {
+            $properties = array('color', 'background-color', 'padding-top');
+            $assert($expected->resolve($element, $properties, array()) === $actual->resolve($element, $properties, array()), 'mixed-arm computed-style parity ' . $count . ' routes: ' . $owner . ' ' . $element->textContent);
+        }
+    }
+    $assert(str_contains($text, ':root:not(') || 2 === $count, 'the dropped rule uses the shorter complementary scope');
+}
+$assert($armSizes[30] < 1.1 * $armSizes[2], 'mixed-arm site-wide bytes stay roughly constant in N', json_encode($armSizes));
+
+// Private richtext carriers (including packed style$/editor-span variants)
+// can be emitted unscoped, but negations and unknown carriers cannot.
+$richOne = 'blocks-engine-richtext-dddddddddddd-0';
+$richTwo = 'blocks-engine-richtext-eeeeeeeeeeee-0';
+$richList = static fn (string $marker): string => ':where(mark[style*="--blocks-engine-richtext-marker:' . $marker . ';"],mark[style$="--blocks-engine-richtext-marker:' . $marker . '"],span[data-blocks-engine-richtext-marker="' . $marker . '"])';
+$richPages = array('one' => array($richList($richOne) . '{color:red}'), 'two' => array($richList($richTwo) . '{color:red}'));
+$richMarkup = array('one' => '<mark style="--blocks-engine-richtext-marker:' . $richOne . ';">One</mark>', 'two' => '<span data-blocks-engine-richtext-marker="' . $richTwo . '">Two</span>');
+$privateRich = $transformer->scopeRouteVariants($richPages['one'], $richPages, array(), null, $richMarkup);
+$assert(!str_contains($privateRich['stylesheet'], 'be-route-'), 'private richtext marker swaps need no route scopes');
+$unknownRich = $transformer->scopeRouteVariants($richPages['one'], $richPages);
+$assert(str_contains($unknownRich['stylesheet'], 'be-route-'), 'missing carrier evidence never proves richtext arms inert');
+$negated = ':not(:where(.blocks-engine-control-aaaaaaaaaaaa-0)){color:red}';
+$negativeMarker = $transformer->scopeRouteVariants(array($negated), array('one' => array($negated), 'two' => array('')), array(), null, array('one' => '', 'two' => ''));
+$assert(str_contains($negativeMarker['stylesheet'], 'be-route-'), 'negated markers are not positive carrier requirements');
+
+// Identical shared units align separately at each source occurrence.
 $marker = 'blocks-engine-control-0123456789ab-0';
 $unit = '.group>.btn{z-index:1}';
 $variant = '.group>.btn,:where(.' . $marker . '){z-index:1}';
-$duplicates = $transformer->foldPositiveMarkerArms(array($unit . '.between{z-index:2}' . $unit), array('a' => array($variant . '.between{z-index:2}' . $variant)), array('a' => '<p class="' . $marker . '">Probe</p>'));
-$assert(2 === substr_count(implode('', $duplicates['shared']), $marker), 'marker folding rewrites every byte-identical shared unit without moving it');
+$duplicates = $transformer->scopeRouteVariants(array($unit . '.between{z-index:2}' . $unit), array('a' => array($variant . '.between{z-index:2}' . $variant)), array(), null, array('a' => '<p class="' . $marker . '">Probe</p>'));
+$assert(2 === substr_count($duplicates['stylesheet'], $marker), 'arm reconciliation rewrites every aligned shared unit without moving it');
 
 if ( 0 < $failures ) {
     fwrite(STDERR, "shared-author-stylesheet scaling FAILED: {$passes} passed, {$failures} failed\n");

From aaa70bd6347c77fa334c67c6aa4ffca0c823eb9d Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Mon, 26 Oct 2026 23:30:34 -0400
Subject: [PATCH 05/10] Leave route variant arms unscoped where outside routes
 cannot match them

---
 .../src/ArtifactCompiler/ArtifactCompiler.php | 23 ++++-
 .../src/Css/CssStylesheetTransformer.php      |  7 +-
 .../src/Css/RouteSelectorMatchIndex.php       | 82 +++++++++++++++++
 .../shared-author-stylesheet-scaling.php      | 87 +++++++++++++++++--
 4 files changed, 185 insertions(+), 14 deletions(-)
 create mode 100644 php-transformer/src/Css/RouteSelectorMatchIndex.php

diff --git a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
index 62c5422f9..ef278143e 100644
--- a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
+++ b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
@@ -2564,11 +2564,30 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
         }
         $transformer = new CssStylesheetTransformer();
         $routeIdentities = array();
+        $routeDocuments = array();
         foreach ($files as $document) {
             if ('html' !== ($document['kind'] ?? '')) continue;
             $source = (string) $document['path'];
-            $routeIdentities[$this->fileOwnership($document)['id']] = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\WordPressSitePlan::identity('page', $source, $source);
+            $owner = $this->fileOwnership($document)['id'];
+            $routeIdentities[$owner] = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\WordPressSitePlan::identity('page', $source, $source);
+            if (!isset($pageMarkup[$owner])) continue;
+            // Parse the lowered markup once per route, never once per arm or
+            // stylesheet. Retain source root/body context just as projection
+            // does; block comments are inert to the HTML parser.
+            $dom = new DOMDocument();
+            $previousErrors = libxml_use_internal_errors(true);
+            $loaded = $dom->loadHTML('<?xml encoding="UTF-8"><html><body>' . $pageMarkup[$owner] . '</body></html>');
+            libxml_clear_errors();
+            libxml_use_internal_errors($previousErrors);
+            if (!$loaded) continue;
+            $attributes = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::metadataFromHtml((string) ($document['content'] ?? ''));
+            foreach (array('html' => 'root_attributes', 'body' => 'body_attributes') as $tag => $key) {
+                $element = $dom->getElementsByTagName($tag)->item(0);
+                foreach ($attributes[$key] ?? array() as $name => $value) $element?->setAttribute($name, $value);
+            }
+            $routeDocuments[$owner] = $dom;
         }
+        $routeMatchIndex = new \Automattic\BlocksEngine\PhpTransformer\Css\RouteSelectorMatchIndex($routeDocuments);
         $reserved = array_fill_keys(array_column($files, 'path'), true);
         $output = array();
         foreach ( $files as $file ) {
@@ -2652,7 +2671,7 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             $sharedStylesheets = array($transformer->concatenateWithoutRedundantRules($sharedStylesheets));
             $fallbackSharedStylesheets = $sharedStylesheets;
             foreach ($pages as $owner => $_) $pages[$owner] = $pageProjections[$owner] ?? $pages[$owner];
-            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $pageProjections, $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup);
+            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $pageProjections, $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup, $routeMatchIndex);
             // Recompact only at serialization, merging compatible marker arms.
             $recompact = static fn (string $css): string => count($pageProjections) > 1 ? CssSelectorListCompactor::stylesheet($css, true) : $css;
             // Bound what route scoping adds to the sheet every route loads by
diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 088168984..029162840 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -268,7 +268,7 @@ private static function positiveMarker(string $arm): ?string
      * @param list<string> $shared
      * @param array<string,list<string>> $pages
      * Aligned rules are expanded into arms, each with its own route set.
-     * Marker arms inert outside that set stay unscoped; all others use the
+     * Arms proven inert outside that set stay unscoped; all others use the
      * shorter positive or complementary negative scope. Only validated lists
      * are split, preserving invalid-selector isolation and source order.
      *
@@ -279,7 +279,7 @@ private static function positiveMarker(string $arm): ?string
      *
      * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>, route_bytes:int}
      */
-    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array()): array
+    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array(), ?RouteSelectorMatchIndex $matchIndex = null): array
     {
         $carriers = self::markerCarriers($markup);
         $base = array();
@@ -369,7 +369,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         $routeClass = static fn ($owner): string => \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($routeIdentities[$owner] ?? (string) $owner);
         $routeBytes = 0;
         $allRoutes = array_fill_keys(array_keys($pages), true);
-        $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $carriers, $markup): void {
+        $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $carriers, $markup, $matchIndex): void {
             [$lead, $rule] = self::splitLeadingComments($prototype['text']);
             $brace = (int) $this->nextRuleBoundary($rule, 0);
             $body = substr($rule, $brace);
@@ -387,6 +387,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                 $outside = array_diff_key($allRoutes, $routes);
                 $marker = self::positiveMarker($arm);
                 $inert = !$outside || (null !== $marker && !array_diff_key($outside, $markup) && !array_diff_key($carriers[$marker] ?? array(), $routes));
+                if (!$inert && null !== $matchIndex) $inert = $matchIndex->inertOutside($arm, $outside);
                 $scope = '';
                 if (!$inert) {
                     $positive = ':where(:root:is(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($routes))) . '))';
diff --git a/php-transformer/src/Css/RouteSelectorMatchIndex.php b/php-transformer/src/Css/RouteSelectorMatchIndex.php
new file mode 100644
index 000000000..675f6ed15
--- /dev/null
+++ b/php-transformer/src/Css/RouteSelectorMatchIndex.php
@@ -0,0 +1,82 @@
+<?php
+declare(strict_types=1);
+
+namespace Automattic\BlocksEngine\PhpTransformer\Css;
+
+use DOMDocument;
+
+/** Immutable compiled documents, indexed once for all author stylesheets. */
+final class RouteSelectorMatchIndex
+{
+    private CssSelectorMatchCache $cache;
+    private array $elements = array();
+    private array $keys = array();
+    private array $selectors = array();
+    private array $hits = array();
+
+    /** @param array<string,DOMDocument> $documents */
+    public function __construct(array $documents)
+    {
+        $this->cache = new CssSelectorMatchCache();
+        foreach ($documents as $owner => $document) {
+            $this->elements[$owner] = array();
+            $this->keys[$owner] = array();
+            foreach ($document->getElementsByTagName('*') as $element) {
+                $this->elements[$owner][] = $element;
+                $keys = array('t:' . strtolower($element->tagName));
+                if ($element->hasAttribute('id')) $keys[] = 'i:' . $element->getAttribute('id');
+                foreach ($this->cache->classTokens($element) as $class) $keys[] = 'c:' . $class;
+                foreach ($keys as $key) $this->keys[$owner][$key][] = $element;
+            }
+        }
+    }
+
+    /** Missing documents and unsupported selectors are never evidence of inertness. */
+    public function inertOutside(string $arm, array $outside): bool
+    {
+        if (!isset($this->selectors[$arm])) {
+            // Match the structural subject in every state, as the projector does.
+            // Do not rewrite quoted attributes or functional negations: those
+            // require reasoning about state alternatives, so retain their scope.
+            $structural = $arm;
+            if (false === strpbrk($arm, '\\"\'') && !str_contains($arm, ':not(')) {
+                $structural = preg_replace('/:(?:hover|focus-visible|focus-within|focus|active|visited|any-link|link|disabled|enabled|target)(?![\w(-])/i', '', $arm) ?? $arm;
+            }
+            $parsed = CssSelectorMatcher::parse($structural);
+            $keys = array();
+            if ($parsed['supported']) {
+                $leaf = $parsed['compounds'][count($parsed['compounds']) - 1];
+                foreach ($leaf['classes'] as $class) $keys[] = 'c:' . $class;
+                // An id also matches an editor anchor carrier: either key
+                // can supply candidates, never require the literal id alone.
+                foreach ($leaf['ids'] as $id) {
+                    $keys[] = array('i:' . $id, 'c:' . \Automattic\BlocksEngine\PhpTransformer\Support\EngineMarker::editorAnchorClass($id));
+                }
+                if (null !== $leaf['type']) $keys[] = 't:' . strtolower($leaf['type']);
+            }
+            $this->selectors[$arm] = array($parsed, $keys);
+        }
+        [$parsed, $keys] = $this->selectors[$arm];
+        if (!$parsed['supported']) return false;
+        foreach ($outside as $owner => $_) {
+            if (!isset($this->elements[$owner])) return false;
+            if (!isset($this->hits[$arm][$owner])) {
+                $candidates = $this->elements[$owner];
+                foreach ($keys as $key) {
+                    $list = array();
+                    foreach ((array) $key as $alternative) $list = array_merge($list, $this->keys[$owner][$alternative] ?? array());
+                    if (count($list) < count($candidates)) $candidates = $list;
+                    if (!$candidates) break;
+                }
+                $hit = false;
+                foreach ($candidates as $element) {
+                    $match = CssSelectorMatcher::matches($element, $parsed, true, $this->cache);
+                    if (!$match['supported'] || $match['matches']) { $hit = true; break; }
+                }
+                $this->hits[$arm][$owner] = $hit;
+            }
+            if ($this->hits[$arm][$owner]) return false;
+        }
+        return true;
+    }
+}
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index a836c0609..971cc17e4 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -336,19 +336,19 @@
     $assert(str_contains($functions, $class) && str_contains($functions, 'blocksEngineDocumentContext') && str_contains($functions, 'language_attributes'), 'the same root registry reaches front end and editor: ' . $row['source_path']);
 }
 $group = ':where(:root:is(.' . $classes['b.html'] . ',.' . $classes['c.html'] . '))';
-$assert(str_contains($routeText, $group . ' .sample'), 'byte-identical constant-arm variants group both span routes');
-$assert(str_contains($routeText, 'html:where(:root:') && str_contains($routeText, ':root:where(:root:'), 'html and root scopes merge into the leftmost compound');
+$assert(str_contains($routeText, '.sample{'), 'plain variants inert on other routes remain shared');
+$assert(str_contains($routeText, 'html .sample') && str_contains($routeText, ':root .sample'), 'inert html and root arms retain their original compounds');
 $droppedOverride = false;
 $conditionalImage = false;
 $transformer->visitStyleRules($routeText, static function (string $prelude, string $body, array $context) use (&$droppedOverride, &$conditionalImage, $classes): void {
     if (str_contains($body, 'padding:7px!important')) {
-        $droppedOverride = in_array('@media(min-width:1px)', $context, true) && str_contains($prelude, ':root:not(.' . $classes['b.html'] . ',.' . $classes['c.html'] . ')');
+        $droppedOverride = in_array('@media(min-width:1px)', $context, true);
     }
-    if (str_contains($prelude, '.wp-block-image') && str_contains($prelude, ':root:is(.' . $classes['e.html'] . ')')) $conditionalImage = in_array('@media(min-width:1px)', $context, true);
+    if (str_contains($prelude, '.wp-block-image')) $conditionalImage = in_array('@media(min-width:1px)', $context, true);
 });
-$assert($droppedOverride, 'the dropped override excludes both span routes at its original conditional position');
+$assert($droppedOverride, 'the inert dropped override retains its original conditional position');
 $assert($conditionalImage, 'page-only image rules retain their at-rule context');
-$assert(str_contains($routeText, ':where(:root:is(.' . $classes['e.html'] . '))'), 'page-only image bridge has a positive route scope');
+$assert(!str_contains($routeText, ':where(:root:is(.' . $classes['e.html'] . '))'), 'page-only image bridge inert elsewhere needs no positive route scope');
 
 // Each route is compared with its independent public compilation, including
 // editor wrapper selectors. Apply exactly the root attributes the theme uses.
@@ -475,8 +475,8 @@
 }
 echo 'Bound fixture author CSS: ' . json_encode(array_map('strlen', $boundAssets)) . "\n";
 
-// The safety net still trips when route-list overhead itself dominates the
-// largest copy, even though no whole selector lists are duplicated anymore.
+// This former safety-net fixture now avoids route-list overhead altogether:
+// absent button/image arms are provably inert on the other routes.
 $budgetFiles = array();
 for ($i = 0; $i < 24; ++$i) {
     $file = $boundFiles[$i % count($boundFiles)];
@@ -489,7 +489,7 @@
 $budgetResult = (new ArtifactCompiler())->compile(array('entrypoint' => 'budget-0.html', 'files' => array_merge($budgetFiles, array($budgetSheet))))->toArray();
 $budgetFindings = array_filter($budgetResult['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '') && str_contains($row['message'], 'largest'));
 $budgetPlan = $budgetResult['source_reports']['wordpress_site_plan'];
-$assert(1 === count($budgetFindings) && str_contains(implode('', array_column($budgetPlan['assets'], 'source_path')), '.page-'), 'excessive route-list overhead retains the sheet-level byte-budget fallback');
+$assert(array() === array_values($budgetFindings) && !str_contains(implode('', array_column($budgetPlan['assets'], 'source_path')), '.page-'), 'inert plain arms avoid the former route-list byte-budget fallback');
 foreach (array_slice($budgetPlan['pages'], 0, 2) as $row) {
     $file = array_column($budgetFiles, null, 'path')[$row['source_path']];
     $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $file['path'], 'files' => array($file, $budgetSheet)))->toArray()['source_reports']['wordpress_site_plan'];
@@ -628,6 +628,75 @@
 $negativeMarker = $transformer->scopeRouteVariants(array($negated), array('one' => array($negated), 'two' => array('')), array(), null, array('one' => '', 'two' => ''));
 $assert(str_contains($negativeMarker['stylesheet'], 'be-route-'), 'negated markers are not positive carrier requirements');
 
+// Plain arms are absent because they do not match, not because of marker
+// ownership. Keep the authored input fixed as N increases. A native-lowered
+// arm absent from a projection but still matching its DOM MUST stay scoped.
+$plainSizes = array();
+$plainStarted = microtime(true);
+$plainArms = array();
+for ($k = 0; $k < 112; ++$k) $plainArms[] = '#page-only-' . $k . ' .x';
+$plainBase = '.native{z-index:9}' . implode(',', $plainArms) . '{color:red}';
+foreach (array(2, 12, 112) as $count) {
+    $plainPages = $plainMarkup = $plainDocuments = array();
+    for ($k = 0; $k < $count; ++$k) {
+        $owner = 'plain-' . $k;
+        $plainPages[$owner] = array(0 === $k ? $plainBase : (1 === $k ? '' : '.native{z-index:9}') . '#page-only-' . $k . ' .x{color:red}');
+        $plainMarkup[$owner] = '<div id="page-only-' . $k . '"><p class="x native">Probe</p></div>';
+        $dom = new DOMDocument();
+        @$dom->loadHTML('<html><body>' . $plainMarkup[$owner] . '</body></html>');
+        $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+        $plainDocuments[$owner] = $dom;
+    }
+    $index = new Automattic\BlocksEngine\PhpTransformer\Css\RouteSelectorMatchIndex($plainDocuments);
+    $plainResult = $transformer->scopeRouteVariants(array($plainBase), $plainPages, array(), null, $plainMarkup, $index);
+    $plainSizes[$count] = strlen($plainResult['stylesheet']);
+    $assert(array() === $plainResult['reasons'] && $count === count($plainResult['scoped']), $count . ' plain routes reconcile without page copies');
+    $transformer->visitStyleRules($plainResult['stylesheet'], static function (string $prelude) use ($assert): void {
+        if (str_contains($prelude, '#page-only-')) $assert(!str_contains($prelude, 'be-route-'), 'plain page-local arms are unscoped');
+        if (str_contains($prelude, '.native')) $assert(str_contains($prelude, 'be-route-'), 'matching native-lowered arm stays scoped');
+    });
+    foreach ($plainDocuments as $owner => $dom) {
+        $element = $dom->getElementsByTagName('p')->item(0);
+        $expected = new StaticCssCascade($dom, implode('', $plainPages[$owner]));
+        $actual = new StaticCssCascade($dom, $plainResult['stylesheet']);
+        $assert($expected->resolve($element, array('color', 'z-index'), array()) === $actual->resolve($element, array('color', 'z-index'), array()), 'plain-arm computed-style parity ' . $owner);
+    }
+}
+$assert($plainSizes[112] < 1.02 * $plainSizes[2], 'plain-arm site-wide bytes stay constant in N', json_encode($plainSizes));
+echo 'Plain-arm fixture: ' . json_encode($plainSizes) . ' bytes; ' . round(microtime(true) - $plainStarted, 3) . " seconds including parity\n";
+// Exercise the public asset path too: unmatched plain arms must not create
+// .page- companions, even when every page retains a different id ancestry.
+$plainAssetSizes = array();
+foreach (array(2, 12) as $count) {
+    $files = array();
+    for ($k = 0; $k < $count; ++$k) $files[] = array('path' => 'plain-' . $k . '.html', 'kind' => 'html', 'content' => '<html><head><link rel="stylesheet" href="base.css"></head><body><main id="page-only-' . $k . '"><p class="x">Probe</p></main></body></html>');
+    $sheet = array('path' => 'base.css', 'kind' => 'css', 'content' => $plainBase);
+    $plan = (new ArtifactCompiler())->compile(array('entrypoint' => 'plain-0.html', 'files' => array_merge($files, array($sheet))))->toArray()['source_reports']['wordpress_site_plan'];
+    $plainAssetSizes[$count] = strlen(implode('', $authoredAssets($plan)));
+    $assert(!str_contains(implode('', array_column($plan['assets'], 'source_path')), '.page-'), $count . ' plain pages emit no page-copy assets');
+    foreach ($plan['pages'] as $row) {
+        $alone = (new ArtifactCompiler())->compile(array('entrypoint' => $row['source_path'], 'files' => array(array_column($files, null, 'path')[$row['source_path']], $sheet)))->toArray()['source_reports']['wordpress_site_plan'];
+        $assert($probeStyles($alone, $alone['pages'][0], false) === $probeStyles($plan, $row, false), 'public plain-arm computed-style parity ' . $row['source_path']);
+    }
+}
+$assert($plainAssetSizes[12] < 1.1 * $plainAssetSizes[2], 'plain-arm published bytes stay roughly constant', json_encode($plainAssetSizes));
+
+// Matching dropped arms still contribute to the unchanged byte safety net.
+$costlyBase = '';
+for ($k = 0; $k < 80; ++$k) $costlyBase .= '.native{z-index:' . $k . '}';
+$costlyPages = array();
+foreach ($plainDocuments as $owner => $_) $costlyPages[$owner] = array(0 === ((int) substr($owner, 6)) % 2 ? $costlyBase : '');
+$costly = $transformer->scopeRouteVariants(array($costlyBase), $costlyPages, array(), null, $plainMarkup, $index);
+$assert($costly['route_bytes'] > 8192 && $costly['route_bytes'] > 3 * strlen($costlyBase), 'matching dropped arms retain over-budget scope accounting');
+
+// Unknown, negated-state and missing-document evidence is conservative.
+foreach (array('.x:unknown', '.x:not(:hover)', '.x:has(> .y)') as $unknownArm) {
+    $assert(!$index->inertOutside($unknownArm, array('plain-1' => true)), 'undecidable arm stays scoped: ' . $unknownArm);
+}
+$assert(!$index->inertOutside('.absent', array('missing' => true)), 'missing document never proves inertness');
+$assert($index->inertOutside('.absent:hover', array('plain-1' => true)), 'state-independent non-match is inert');
+$assert(!$index->inertOutside('.x:hover', array('plain-1' => true)), 'stateful structural match stays scoped');
+
 // Identical shared units align separately at each source occurrence.
 $marker = 'blocks-engine-control-0123456789ab-0';
 $unit = '.group>.btn{z-index:1}';

From 0dd90c71495e2f3d20a251be2d67e01d1da9e1e4 Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Tue, 27 Oct 2026 01:06:17 -0400
Subject: [PATCH 06/10] Keep shared @import site-wide, shorten route classes,
 budget route bytes by total copy cost

---
 .../src/ArtifactCompiler/ArtifactCompiler.php | 54 ++++++++-----
 .../src/Css/CssStylesheetTransformer.php      | 38 ++++++++-
 .../WordPressSitePlan/DocumentRootContext.php | 41 +++++++++-
 .../shared-author-stylesheet-scaling.php      | 77 +++++++++++++++++--
 .../unit/document-root-font-ownership.php     |  2 +-
 5 files changed, 178 insertions(+), 34 deletions(-)

diff --git a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
index ef278143e..f4b93d79d 100644
--- a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
+++ b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
@@ -52,16 +52,17 @@ final class ArtifactCompiler
     public const MAX_PLAN_DIGEST_DEPTH = 512;
 
     /**
-     * Route-scoped variants live in the one sheet every route downloads. When
-     * the bytes they add (scoped variants and page-only rules, plus the growth
-     * of shared rules that exclude routes) exceed this factor times the
-     * largest page copy (`<sheet>.page-<hash>.css`, what one page would load
-     * on top of the shared sheet), that stylesheet falls back to page copies
-     * with an `author_stylesheet_route_scope_fallback` diagnostic. A visitor
-     * then never loads more than about three times the extra CSS of a copy.
+     * The shared sheet is downloaded once and cached across routes; page
+     * copies cost a separate download per page. Allow the greater of this
+     * factor times the largest copy, one tenth of total copy bytes, and the
+     * floor below. Measure scoped variants and split-rule overhead before
+     * compaction; exceeding the bound falls back to page copies.
      */
     public const ROUTE_SCOPE_BYTES_FACTOR = 3;
 
+    /** Cached shared overhead may replace up to 10% of total per-page copy download bytes. */
+    public const ROUTE_SCOPE_COPY_TOTAL_DIVISOR = 10;
+
     /** Route-scoping overhead always allowed, e.g. for routes that only drop rules (their copy is empty). */
     public const ROUTE_SCOPE_BYTES_FLOOR = 8192;
 
@@ -87,6 +88,9 @@ final class ArtifactCompiler
     /** @var array<string, list<array<string, mixed>>> */
     private array $stylesheetOccurrenceCache = array();
 
+    /** @var array<string,string> Collision-free root classes keyed by site route identity. */
+    private array $routeClasses = array();
+
     /** @var array<string, true> Source resources imported into an authored layer. */
     private array $layeredNavigationImports = array();
 
@@ -289,6 +293,13 @@ private function finalizeArtifact(array $artifact, array $reduction): Transforme
     {
         $startedAt = hrtime(true);
         $normalized = $reduction['normalized'];
+        $routeIdentities = array();
+        foreach ($normalized['files'] as $file) {
+            if ('html' !== ($file['kind'] ?? '')) continue;
+            $source = (string) $file['path'];
+            $routeIdentities[] = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\WordPressSitePlan::identity('page', $source, $source);
+        }
+        $this->routeClasses = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClasses($routeIdentities);
         $capturedDialogs = is_array($reduction['captured_dialogs'] ?? null) ? $reduction['captured_dialogs'] : array('diagnostics' => array(), 'projected_count' => 0);
         $entry = $this->entryFile($normalized['files'], $normalized['entrypoints']);
         $documents = is_array($reduction['source_documents'] ?? null) ? $reduction['source_documents'] : $this->compileSourceDocuments($normalized);
@@ -2645,11 +2656,13 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             $stylesheets = array();
             foreach ( $authoritativeContent as $stylesheet ) {
                 $split = $transformer->splitLeadingAtRulePreamble($stylesheet);
-                if ( '' !== trim($split['preamble']) ) {
-                    $preambles[] = $split['preamble'];
+                $imports = '';
+                foreach ($split['statements'] as $statement) {
+                    if (preg_match('/^\s*(?:\/\*.*?\*\/\s*)*@import\b/is', $statement)) $imports .= $statement;
+                    else $preambles[] = $statement;
                 }
-                if ( '' !== trim($split['stylesheet']) ) {
-                    $stylesheets[] = $split['stylesheet'];
+                if ( '' !== trim($imports . $split['stylesheet']) ) {
+                    $stylesheets[] = $imports . $split['stylesheet'];
                 }
             }
             $sharedStylesheets = array_merge(array_map('strval', array_keys($sharedByPath[$path] ?? array())), $stylesheets);
@@ -2671,21 +2684,22 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             $sharedStylesheets = array($transformer->concatenateWithoutRedundantRules($sharedStylesheets));
             $fallbackSharedStylesheets = $sharedStylesheets;
             foreach ($pages as $owner => $_) $pages[$owner] = $pageProjections[$owner] ?? $pages[$owner];
-            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $pageProjections, $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup, $routeMatchIndex);
+            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $pageProjections, $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup, $routeMatchIndex, $this->routeClasses);
             // Recompact only at serialization, merging compatible marker arms.
             $recompact = static fn (string $css): string => count($pageProjections) > 1 ? CssSelectorListCompactor::stylesheet($css, true) : $css;
-            // Bound what route scoping adds to the sheet every route loads by
-            // what the largest page copy would have added to one page. Both
-            // are measured before compaction.
-            $largestCopy = 0;
+            // A cached shared download replaces separate page-copy downloads.
+            // Both costs are measured before compaction.
+            $largestCopy = $totalCopyBytes = 0;
             if (0 < $routed['route_bytes']) {
                 foreach ($routed['scoped'] as $owner) {
-                    $largestCopy = max($largestCopy, strlen($transformer->rulesAbsentFrom($pageProjections[$owner] ?? array(), $fallbackSharedStylesheets, true)));
+                    $copyBytes = strlen($transformer->rulesAbsentFrom($pageProjections[$owner] ?? array(), $fallbackSharedStylesheets, true));
+                    $largestCopy = max($largestCopy, $copyBytes);
+                    $totalCopyBytes += $copyBytes;
                 }
             }
-            $routeBudget = max(self::ROUTE_SCOPE_BYTES_FACTOR * $largestCopy, self::ROUTE_SCOPE_BYTES_FLOOR);
+            $routeBudget = max(self::ROUTE_SCOPE_BYTES_FACTOR * $largestCopy, intdiv($totalCopyBytes, self::ROUTE_SCOPE_COPY_TOTAL_DIVISOR), self::ROUTE_SCOPE_BYTES_FLOOR);
             if (array() !== $routed['scoped'] && $routed['route_bytes'] > $routeBudget) {
-                $projectionDiagnostics[] = $this->diagnostic('author_stylesheet_route_scope_fallback', 'warning', sprintf('%s: route-scoped variants add %d bytes to every route, over %d (%dx the largest %d-byte page copy); emitting page copies', $path, $routed['route_bytes'], $routeBudget, self::ROUTE_SCOPE_BYTES_FACTOR, $largestCopy));
+                $projectionDiagnostics[] = $this->diagnostic('author_stylesheet_route_scope_fallback', 'warning', sprintf('%s: route-scoped variants add %d cached-sheet bytes, over budget %d = max(%dx largest copy %d, total copies %d / %d, floor %d); emitting page copies', $path, $routed['route_bytes'], $routeBudget, self::ROUTE_SCOPE_BYTES_FACTOR, $largestCopy, $totalCopyBytes, self::ROUTE_SCOPE_COPY_TOTAL_DIVISOR, self::ROUTE_SCOPE_BYTES_FLOOR));
                 foreach ($routed['scoped'] as $owner) $pages[$owner] ??= $pageProjections[$owner] ?? array();
                 $routed['stylesheet'] = implode("\n", $fallbackSharedStylesheets);
             } else {
@@ -4226,7 +4240,7 @@ private function fullDocumentMetadata(string $html, string $sourcePath, array $f
         $titles = HtmlTagScanner::scan($html, 'title');
         $title = isset($titles[0]) ? trim(html_entity_decode(strip_tags($titles[0]['content']), ENT_QUOTES | ENT_HTML5, 'UTF-8')) : $this->titleFromHtml($html, $sourcePath);
         $routeIdentity = \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\WordPressSitePlan::identity('page', $sourcePath, $sourcePath);
-        $metadata = array('source_context' => array('source_path' => $sourcePath, 'kind' => 'html'), ...\Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::metadataFromHtml($html, $routeIdentity), 'title' => $title, 'title_declaration' => array('order' => 0, 'placement' => 'head'), 'meta' => $meta, 'links' => $links, 'scripts' => $scripts);
+        $metadata = array('source_context' => array('source_path' => $sourcePath, 'kind' => 'html'), ...\Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::metadataFromHtml($html, $routeIdentity, $this->routeClasses[$routeIdentity] ?? null), 'title' => $title, 'title_declaration' => array('order' => 0, 'placement' => 'head'), 'meta' => $meta, 'links' => $links, 'scripts' => $scripts);
         if ( array() !== $projectedRootMarkers ) {
             $rootClasses = array_values(array_filter(preg_split('/\s+/', trim((string) ($metadata['root_attributes']['class'] ?? ''))) ?: array(), static fn(string $class): bool => '' !== $class));
             $metadata['root_attributes']['class'] = implode(' ', array_unique(array_merge($rootClasses, $projectedRootMarkers)));
diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 029162840..656885d9c 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -264,6 +264,8 @@ private static function positiveMarker(string $arm): ?string
      * declaration references them, so the ones only some routes keep or add
      * stay site-wide, once each. Two routes that define the same name with
      * different bodies would see each other's definition, so they fall back.
+     * Leading @imports stay shared only when every route retains the exact
+     * same ordered text; differing import lists use the copy fallback.
      *
      * @param list<string> $shared
      * @param array<string,list<string>> $pages
@@ -274,16 +276,18 @@ private static function positiveMarker(string $arm): ?string
      *
      * @param array<string,string> $routeIdentities Route identities by compilation owner.
      * @param array<string,string> $markup Canonical markup by compilation owner.
+     * @param array<string,string> $routeClasses Site-wide collision-free classes by route identity.
      * `route_bytes` counts scope and split-rule overhead beyond the unscoped
      * arm union, plus scoped insertions, before final compaction.
      *
      * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>, route_bytes:int}
      */
-    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array(), ?RouteSelectorMatchIndex $matchIndex = null): array
+    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array(), ?RouteSelectorMatchIndex $matchIndex = null, array $routeClasses = array()): array
     {
         $carriers = self::markerCarriers($markup);
         $base = array();
         foreach ($shared as $css) $this->collectRuleUnits($css, array(), $base);
+        $baseImports = self::routeImports($base);
         $normal = static fn (array $unit): string => trim(CssSyntaxScanner::withoutComments($unit['text']));
         $key = function (array $unit) use ($normal): string {
             $rule = self::splitLeadingComments($unit['text'])[1];
@@ -311,6 +315,10 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             }
             if (null !== $reason) { $reasons[$owner] = $reason; continue; }
             foreach ($sheets as $css) $this->collectRuleUnits($css, array(), $units);
+            if (null === $baseImports || self::routeImports($units) !== $baseImports) {
+                $reasons[$owner] = 'different or non-leading @import rules';
+                continue;
+            }
             $cursor = 0;
             $localChanges = $localInsertions = $localSite = array();
             foreach ($units as $unit) {
@@ -366,7 +374,10 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             }
         }
         $output = array();
-        $routeClass = static fn ($owner): string => \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($routeIdentities[$owner] ?? (string) $owner);
+        $identities = array();
+        foreach (array_keys($pages) as $owner) $identities[] = $routeIdentities[$owner] ?? (string) $owner;
+        $routeClasses = $routeClasses ?: \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClasses($identities);
+        $routeClass = static fn ($owner): string => $routeClasses[$routeIdentities[$owner] ?? (string) $owner];
         $routeBytes = 0;
         $allRoutes = array_fill_keys(array_keys($pages), true);
         $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $carriers, $markup, $matchIndex): void {
@@ -434,6 +445,29 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         return array('stylesheet' => $this->serializeRuleUnits($output), 'scoped' => $scoped, 'reasons' => $reasons, 'route_bytes' => $routeBytes);
     }
 
+    /**
+     * Imports must match byte-for-byte, in order, at the sheet top. Unlike
+     * definitions they cannot be added or dropped independently by a route.
+     *
+     * @param list<array{context:list<string>,text:string,dedupable:bool}> $units
+     * @return list<string>|null
+     */
+    private static function routeImports(array $units): ?array
+    {
+        $imports = array();
+        $top = true;
+        foreach ($units as $unit) {
+            [, $rule] = self::splitLeadingComments($unit['text']);
+            if (preg_match('/^@import\b/i', $rule)) {
+                if (!$top || $unit['context']) return null;
+                $imports[] = $unit['text'];
+            } elseif ('' !== trim($rule) && !preg_match('/^@charset\b/i', $rule)) {
+                $top = false;
+            }
+        }
+        return $imports;
+    }
+
     /**
      * The name a route-independent definition at-rule binds, or null for any
      * other unit. Same-name definitions with different bodies conflict.
diff --git a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
index a054f60e1..292d54933 100644
--- a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
+++ b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
@@ -17,17 +17,50 @@ public static function fromHtml(string $html, string $tagName = 'html'): array
     }
 
     /** @return array{root_attributes:array<string,string>,body_attributes:array<string,string>} */
-    public static function metadataFromHtml(string $html, string $routeIdentity = ''): array
+    public static function metadataFromHtml(string $html, string $routeIdentity = '', ?string $routeClass = null): array
     {
         $attributes = DocumentRootAttributes::forDocument($html);
-        if ('' !== $routeIdentity) $attributes['html']['class'] = trim(($attributes['html']['class'] ?? '') . ' ' . self::routeClass($routeIdentity));
+        if ('' !== $routeIdentity) $attributes['html']['class'] = trim(($attributes['html']['class'] ?? '') . ' ' . ($routeClass ?? self::routeClass($routeIdentity)));
         return array('root_attributes' => $attributes['html'], 'body_attributes' => $attributes['body']);
     }
 
     /** Stable per-document route class from the compile-time source identity, used by CSS and the root registry alike. */
-    public static function routeClass(string $identity): string
+    public static function routeClass(string $identity, int $length = 8): string
     {
-        return 'be-route-' . substr(hash('sha256', $identity), 0, 12);
+        return 'be-r-' . substr(hash('sha256', $identity), 0, $length);
+    }
+
+    /**
+     * Allocate deterministic classes for the whole site, extending only colliding
+     * hashes. Sorting makes the result independent of route discovery order.
+     *
+     * @param list<string> $identities
+     * @return array<string,string> Classes keyed by route identity.
+     */
+    public static function routeClasses(array $identities): array
+    {
+        $identities = array_values(array_unique($identities));
+        sort($identities);
+        $lengths = array_fill_keys($identities, 8);
+        do {
+            $groups = $classes = array();
+            foreach ($identities as $identity) {
+                $class = self::routeClass($identity, $lengths[$identity]);
+                $classes[$identity] = $class;
+                $groups[$class][] = $identity;
+            }
+            $collision = false;
+            foreach ($groups as $group) {
+                if (count($group) < 2) continue;
+                foreach ($group as $identity) {
+                    if ($lengths[$identity] >= 64) throw new \LogicException('Route identity hash collision.');
+                    ++$lengths[$identity];
+                }
+                $collision = true;
+            }
+        } while ($collision);
+        if (count(array_unique($classes)) !== count($identities)) throw new \LogicException('Route classes must be unique.');
+        return $classes;
     }
 
     public static function assertValid(mixed $attributes): void
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index 971cc17e4..c7e983d18 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -525,7 +525,7 @@
     $text = implode('', $site['assets']);
     $markerSizes[$count] = strlen($text);
     $assert(1 === count($site['assets']) && !str_contains(implode('', array_column($site['plan']['assets'], 'source_path')), '.page-'), $count . ' marker-only pages: one site-wide asset, no page copies', json_encode(array_keys($site['assets'])));
-    $assert(!str_contains($text, 'be-route-'), $count . ' marker-only pages: nothing is route scoped', $text);
+    $assert(!str_contains($text, 'be-r-'), $count . ' marker-only pages: nothing is route scoped', $text);
     $assert(array() === array_filter($site['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '')), $count . ' marker-only pages: no route-scope fallback');
 }
 $markerText = implode('', $site['assets']);
@@ -559,7 +559,7 @@
 $gapText = implode('', $gapSite['assets']);
 $scopedMarkerRule = false;
 $transformer->visitStyleRules($gapText, static function (string $prelude) use (&$scopedMarkerRule): void {
-    if (str_contains($prelude, 'be-route-') && preg_match('/blocks-engine-control-[a-f0-9]{12}-\d+/', $prelude)) $scopedMarkerRule = true;
+    if (str_contains($prelude, 'be-r-') && preg_match('/blocks-engine-control-[a-f0-9]{12}-\d+/', $prelude)) $scopedMarkerRule = true;
 });
 $assert(1 === count($gapSite['assets']) && !$scopedMarkerRule, 'page-only marker rules fold into unscoped unions', $gapText);
 foreach (array(array($site, false), array($gapSite, true)) as [$checked, $gaps]) {
@@ -621,12 +621,12 @@
 $richPages = array('one' => array($richList($richOne) . '{color:red}'), 'two' => array($richList($richTwo) . '{color:red}'));
 $richMarkup = array('one' => '<mark style="--blocks-engine-richtext-marker:' . $richOne . ';">One</mark>', 'two' => '<span data-blocks-engine-richtext-marker="' . $richTwo . '">Two</span>');
 $privateRich = $transformer->scopeRouteVariants($richPages['one'], $richPages, array(), null, $richMarkup);
-$assert(!str_contains($privateRich['stylesheet'], 'be-route-'), 'private richtext marker swaps need no route scopes');
+$assert(!str_contains($privateRich['stylesheet'], 'be-r-'), 'private richtext marker swaps need no route scopes');
 $unknownRich = $transformer->scopeRouteVariants($richPages['one'], $richPages);
-$assert(str_contains($unknownRich['stylesheet'], 'be-route-'), 'missing carrier evidence never proves richtext arms inert');
+$assert(str_contains($unknownRich['stylesheet'], 'be-r-'), 'missing carrier evidence never proves richtext arms inert');
 $negated = ':not(:where(.blocks-engine-control-aaaaaaaaaaaa-0)){color:red}';
 $negativeMarker = $transformer->scopeRouteVariants(array($negated), array('one' => array($negated), 'two' => array('')), array(), null, array('one' => '', 'two' => ''));
-$assert(str_contains($negativeMarker['stylesheet'], 'be-route-'), 'negated markers are not positive carrier requirements');
+$assert(str_contains($negativeMarker['stylesheet'], 'be-r-'), 'negated markers are not positive carrier requirements');
 
 // Plain arms are absent because they do not match, not because of marker
 // ownership. Keep the authored input fixed as N increases. A native-lowered
@@ -652,8 +652,8 @@
     $plainSizes[$count] = strlen($plainResult['stylesheet']);
     $assert(array() === $plainResult['reasons'] && $count === count($plainResult['scoped']), $count . ' plain routes reconcile without page copies');
     $transformer->visitStyleRules($plainResult['stylesheet'], static function (string $prelude) use ($assert): void {
-        if (str_contains($prelude, '#page-only-')) $assert(!str_contains($prelude, 'be-route-'), 'plain page-local arms are unscoped');
-        if (str_contains($prelude, '.native')) $assert(str_contains($prelude, 'be-route-'), 'matching native-lowered arm stays scoped');
+        if (str_contains($prelude, '#page-only-')) $assert(!str_contains($prelude, 'be-r-'), 'plain page-local arms are unscoped');
+        if (str_contains($prelude, '.native')) $assert(str_contains($prelude, 'be-r-'), 'matching native-lowered arm stays scoped');
     });
     foreach ($plainDocuments as $owner => $dom) {
         $element = $dom->getElementsByTagName('p')->item(0);
@@ -704,6 +704,69 @@
 $duplicates = $transformer->scopeRouteVariants(array($unit . '.between{z-index:2}' . $unit), array('a' => array($variant . '.between{z-index:2}' . $variant)), array(), null, array('a' => '<p class="' . $marker . '">Probe</p>'));
 $assert(2 === substr_count($duplicates['stylesheet'], $marker), 'arm reconciliation rewrites every aligned shared unit without moving it');
 
+// Identical leading imports remain site-wide despite non-dedupable units.
+$import = '@import url(/external/8e2e7efb6cf0362d/css-659beb8e13af.css);';
+$importSheet = $boundSheet;
+$importSheet['content'] = $import . $boundSheet['content'];
+$importResult = (new ArtifactCompiler())->compile(array('entrypoint' => 'bound-0.html', 'files' => array_merge($boundFiles, array($importSheet))))->toArray();
+$importPlan = $importResult['source_reports']['wordpress_site_plan'];
+$assert(!str_contains(implode('', array_column($importPlan['assets'], 'source_path')), '.page-'), 'identical leading imports emit no page copies');
+$assert(1 === substr_count(implode('', $authoredAssets($importPlan)), $import), 'identical import stays site-wide once');
+foreach (array('@import url(other.css);', '', $import . '@import url(extra.css);') as $otherImport) {
+    $result = $transformer->scopeRouteVariants(array($import . '.a{color:red}'), array('a' => array($otherImport . '.a{color:blue}')));
+    $assert(isset($result['reasons']['a']), 'different import lists fall back');
+}
+$result = $transformer->scopeRouteVariants(array($import . '@import url(second.css);.a{color:red}'), array('a' => array('@import url(second.css);' . $import . '.a{color:blue}')));
+$assert(isset($result['reasons']['a']), 'reordered imports fall back');
+
+$rootContext = Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::class;
+$classes = array();
+foreach ($importPlan['pages'] as $row) {
+    preg_match('/\\bbe-r-[a-f0-9]+\\b/', $row['document_metadata']['root_attributes']['class'], $match);
+    $classes[] = $match[0] ?? '';
+}
+$assert(count(array_unique($classes)) === count($classes) && !in_array('', $classes, true), 'route classes are unique across the plan');
+$assert(array(13) === array_values(array_unique(array_map('strlen', $classes))), 'normal route classes use the readable eight-hex minimum');
+$collisionIdentities = array('collision-55045', 'collision-70885', 'ordinary-route');
+$collisionClasses = $rootContext::routeClasses($collisionIdentities);
+$assert(3 === count(array_unique($collisionClasses)) && 14 === strlen($collisionClasses['collision-55045']) && 13 === strlen($collisionClasses['ordinary-route']), 'colliding hashes lengthen only as needed');
+$assert($collisionClasses === $rootContext::routeClasses(array_reverse($collisionIdentities)), 'class allocation is independent of route order');
+
+// Exercise the compiler budget with explicit projections: all routes carry
+// the subject, so dropping a rule cannot be proven inert. Long unchanged
+// cascade tails make legacy copies costly in aggregate, not individually.
+$copyBase = '.native{z-index:1}';
+for ($k = 0; $k < 160; ++$k) $copyBase .= '.tail-' . $k . '{z-index:2;color:#123456;margin:0;padding:0}';
+$copyVariant = str_replace('.native{z-index:1}', '.native{z-index:3}', $copyBase);
+$copyPages = $copyFiles = $copyProjections = $copyMarkup = array();
+for ($k = 0; $k < 112; ++$k) {
+    $owner = 'copy-' . $k;
+    $copyPages[$owner] = array($k % 2 ? $copyVariant : $copyBase);
+    $copyMarkup[$owner] = '<div class="native">Probe</div>';
+    $copyProjections[] = array('path' => 'base.css', 'content' => $copyPages[$owner][0], 'owner' => $owner);
+}
+// Many repeated differing units make the scope overhead exceed the old 3x
+// limit while the shared cascade tail dominates each complete page copy.
+$copyBase = str_repeat('.native{z-index:1}', 10) . $copyBase;
+$copyVariant = str_replace('z-index:1}', 'z-index:3}', $copyBase);
+foreach ($copyPages as $owner => &$sheets) $sheets = array((int) substr($owner, 5) % 2 ? $copyVariant : $copyBase);
+unset($sheets);
+foreach ($copyProjections as &$projection) $projection['content'] = $copyPages[$projection['owner']][0];
+unset($projection);
+$copyRoute = $transformer->scopeRouteVariants(array($copyBase), $copyPages, array(), null, $copyMarkup);
+$totalCopies = $largestCopy = 0;
+foreach ($copyPages as $sheets) {
+    $bytes = strlen($transformer->rulesAbsentFrom($sheets, array($copyBase), true));
+    $totalCopies += $bytes;
+    $largestCopy = max($largestCopy, $bytes);
+}
+$assert($copyRoute['route_bytes'] > max(8192, 3 * $largestCopy) && $copyRoute['route_bytes'] <= intdiv($totalCopies, 10), 'total-copy budget accepts overhead rejected by the old largest-copy bound', json_encode(array($copyRoute['route_bytes'], $largestCopy, $totalCopies)));
+$copyFiles[] = array('path' => 'base.css', 'kind' => 'css', 'content' => $copyBase);
+$method = new ReflectionMethod(ArtifactCompiler::class, 'applyAuthorStylesheetProjections');
+$copyDiagnostics = array();
+$copyOutput = $method->invokeArgs(new ArtifactCompiler(), array($copyFiles, $copyProjections, array(array('path' => 'base.css', 'content' => $copyBase)), '', $copyMarkup, &$copyDiagnostics));
+$assert(!str_contains(implode('', array_column($copyOutput, 'path')), '.page-') && !$copyDiagnostics, 'compiler applies total-copy budget without fallback', json_encode($copyDiagnostics));
+
 if ( 0 < $failures ) {
     fwrite(STDERR, "shared-author-stylesheet scaling FAILED: {$passes} passed, {$failures} failed\n");
     exit(1);
diff --git a/php-transformer/tests/unit/document-root-font-ownership.php b/php-transformer/tests/unit/document-root-font-ownership.php
index 9b9af3634..69765057a 100644
--- a/php-transformer/tests/unit/document-root-font-ownership.php
+++ b/php-transformer/tests/unit/document-root-font-ownership.php
@@ -23,7 +23,7 @@
 $assert('document' === ($pages['index.html']['document_metadata']['root_attributes']['id'] ?? null), 'root ID travels alongside classes');
 $bootstrap = '';
 foreach ($plan['writes'] as $write) if ('functions.php' === $write['target_path']) $bootstrap = $write['payload']['data'];
-$assert(str_contains($bootstrap, "'language_attributes'") && str_contains($bootstrap, 'WP_HTML_Tag_Processor') && str_contains($bootstrap, 'type-a be-route-') && str_contains($bootstrap, 'type-b be-route-'), 'generated runtime adopts route-owned root identity through native HTML API');
+$assert(str_contains($bootstrap, "'language_attributes'") && str_contains($bootstrap, 'WP_HTML_Tag_Processor') && str_contains($bootstrap, 'type-a be-r-') && str_contains($bootstrap, 'type-b be-r-'), 'generated runtime adopts route-owned root identity through native HTML API');
 $assert(str_contains($bootstrap, "'_blocks_engine_reconciliation_identity'") && str_contains($bootstrap, 'is_front_page()'), 'runtime selection uses destination identity with route fallback');
 $css = implode("\n", array_column(array_filter($plan['assets'], static fn(array $asset): bool => 'css' === $asset['kind']), 'content'));
 $assert(str_contains($css, '.type-a') && str_contains($css, '.type-b') && str_contains($css, '@media'), 'authored root selector and conditional cascade stay intact');

From b03d1a8c961b28abdff73da55bf053b505184ce2 Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Tue, 27 Oct 2026 01:48:07 -0400
Subject: [PATCH 07/10] Ignore @import rules CSS drops after other rules when
 route scoping

---
 .../src/Css/CssStylesheetTransformer.php      | 25 +++++++++++++++++++
 .../shared-author-stylesheet-scaling.php      |  4 +++
 2 files changed, 29 insertions(+)

diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 656885d9c..eb24a550a 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -287,6 +287,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         $carriers = self::markerCarriers($markup);
         $base = array();
         foreach ($shared as $css) $this->collectRuleUnits($css, array(), $base);
+        $base = self::withoutIgnoredImports($base);
         $baseImports = self::routeImports($base);
         $normal = static fn (array $unit): string => trim(CssSyntaxScanner::withoutComments($unit['text']));
         $key = function (array $unit) use ($normal): string {
@@ -315,6 +316,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             }
             if (null !== $reason) { $reasons[$owner] = $reason; continue; }
             foreach ($sheets as $css) $this->collectRuleUnits($css, array(), $units);
+            $units = self::withoutIgnoredImports($units);
             if (null === $baseImports || self::routeImports($units) !== $baseImports) {
                 $reasons[$owner] = 'different or non-leading @import rules';
                 continue;
@@ -468,6 +470,29 @@ private static function routeImports(array $units): ?array
         return $imports;
     }
 
+    /**
+     * Drop top-level @import rules that follow other rules. CSS ignores them,
+     * so they are inert on every route; concatenated sheets often repeat them.
+     *
+     * @param list<array{context:list<string>,text:string,dedupable:bool}> $units
+     * @return list<array{context:list<string>,text:string,dedupable:bool}>
+     */
+    private static function withoutIgnoredImports(array $units): array
+    {
+        $top = true;
+        $kept = array();
+        foreach ($units as $unit) {
+            [, $rule] = self::splitLeadingComments($unit['text']);
+            if (preg_match('/^@import\b/i', $rule) && !$unit['context']) {
+                if ($top) $kept[] = $unit;
+                continue;
+            }
+            if ('' !== trim($rule) && !preg_match('/^@charset\b/i', $rule)) $top = false;
+            $kept[] = $unit;
+        }
+        return $kept;
+    }
+
     /**
      * The name a route-independent definition at-rule binds, or null for any
      * other unit. Same-name definitions with different bodies conflict.
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index c7e983d18..1f594cac1 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -718,6 +718,10 @@
 }
 $result = $transformer->scopeRouteVariants(array($import . '@import url(second.css);.a{color:red}'), array('a' => array('@import url(second.css);' . $import . '.a{color:blue}')));
 $assert(isset($result['reasons']['a']), 'reordered imports fall back');
+// CSS ignores @import after other rules, so repeated trailing imports (concatenated sheets) are inert on every route.
+$repeated = $import . '.a{color:red}' . $import;
+$result = $transformer->scopeRouteVariants(array($repeated), array('a' => array($import . '.a{color:blue}' . $import)));
+$assert(!isset($result['reasons']['a']) && 1 === substr_count($result['stylesheet'], $import), 'ignored trailing imports neither block route scoping nor survive');
 
 $rootContext = Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::class;
 $classes = array();

From e0393f3e15e79e824fc2d35d92f3947198ba66bb Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Tue, 27 Oct 2026 09:38:28 -0400
Subject: [PATCH 08/10] Keep render-time route state and non-plan pages correct
 under route scoping

---
 .../src/Css/CssStylesheetTransformer.php      | 25 +++--
 .../src/Css/RouteSelectorMatchIndex.php       | 20 ++++
 .../shared-author-stylesheet-scaling.php      | 96 ++++++++++++++-----
 3 files changed, 110 insertions(+), 31 deletions(-)

diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index eb24a550a..ece2798ac 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -271,7 +271,8 @@ private static function positiveMarker(string $arm): ?string
      * @param array<string,list<string>> $pages
      * Aligned rules are expanded into arms, each with its own route set.
      * Arms proven inert outside that set stay unscoped; all others use the
-     * shorter positive or complementary negative scope. Only validated lists
+     * positive scope for variant-only arms, or a complementary negative scope
+     * for arms retained by the virtual no-route-class base document. Only validated lists
      * are split, preserving invalid-selector isolation and source order.
      *
      * @param array<string,string> $routeIdentities Route identities by compilation owner.
@@ -382,7 +383,12 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         $routeClass = static fn ($owner): string => $routeClasses[$routeIdentities[$owner] ?? (string) $owner];
         $routeBytes = 0;
         $allRoutes = array_fill_keys(array_keys($pages), true);
-        $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $carriers, $markup, $matchIndex): void {
+        // Non-plan WordPress pages receive the unprojected shared sheet. They
+        // have no route class and no compiled markup, so can never prove inertness.
+        $baseOwner = '';
+        while (isset($allRoutes[$baseOwner])) $baseOwner .= '#';
+        $allRoutes[$baseOwner] = true;
+        $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $baseOwner, $carriers, $markup, $matchIndex): void {
             [$lead, $rule] = self::splitLeadingComments($prototype['text']);
             $brace = (int) $this->nextRuleBoundary($rule, 0);
             $body = substr($rule, $brace);
@@ -399,13 +405,18 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             foreach ($members as $arm => $routes) {
                 $outside = array_diff_key($allRoutes, $routes);
                 $marker = self::positiveMarker($arm);
-                $inert = !$outside || (null !== $marker && !array_diff_key($outside, $markup) && !array_diff_key($carriers[$marker] ?? array(), $routes));
-                if (!$inert && null !== $matchIndex) $inert = $matchIndex->inertOutside($arm, $outside);
+                $inert = !$outside;
+                if (!$inert && !isset($outside[$baseOwner]) && !RouteSelectorMatchIndex::dependsOnRouteState($arm)) {
+                    $inert = null !== $marker && !array_diff_key($outside, $markup) && !array_diff_key($carriers[$marker] ?? array(), $routes);
+                    if (!$inert && null !== $matchIndex) $inert = $matchIndex->inertOutside($arm, $outside);
+                }
                 $scope = '';
                 if (!$inert) {
-                    $positive = ':where(:root:is(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($routes))) . '))';
-                    $negative = ':where(:root:not(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($outside))) . '))';
-                    $scope = strlen($positive) <= strlen($negative) ? $positive : $negative;
+                    if (isset($routes[$baseOwner])) {
+                        $scope = ':where(:root:not(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($outside))) . '))';
+                    } else {
+                        $scope = ':where(:root:is(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($routes))) . '))';
+                    }
                 }
                 $groups[$scope][] = $arm;
             }
diff --git a/php-transformer/src/Css/RouteSelectorMatchIndex.php b/php-transformer/src/Css/RouteSelectorMatchIndex.php
index 675f6ed15..bece7998c 100644
--- a/php-transformer/src/Css/RouteSelectorMatchIndex.php
+++ b/php-transformer/src/Css/RouteSelectorMatchIndex.php
@@ -31,9 +31,29 @@ public function __construct(array $documents)
         }
     }
 
+    /**
+     * Compiled markup cannot disprove state WordPress restores at render time.
+     * Use the same class vocabulary as RouteStateProjector's route binding,
+     * including Core's current-menu/current_page classes and BE's current item.
+     * Scan the whole arm, including functional pseudo-class arguments.
+     */
+    public static function dependsOnRouteState(string $arm): bool
+    {
+        if (preg_match('/\[\s*(?:aria-current|data-status|data-state)\b/i', $arm)) return true;
+        // Escapes cannot safely be interpreted by this lightweight proof.
+        if (str_contains($arm, '\\')) return true;
+        preg_match_all('/\.([A-Za-z0-9_-]+)/', $arm, $classes);
+        foreach ($classes[1] as $class) {
+            if (\Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\Classification\MenuVocabulary::containsCurrentStateToken($class)) return true;
+        }
+        // Class attribute selectors may name the same mutable state tokens.
+        return 1 === preg_match('/\[\s*class\b/i', $arm);
+    }
+
     /** Missing documents and unsupported selectors are never evidence of inertness. */
     public function inertOutside(string $arm, array $outside): bool
     {
+        if (self::dependsOnRouteState($arm)) return false;
         if (!isset($this->selectors[$arm])) {
             // Match the structural subject in every state, as the projector does.
             // Do not rewrite quoted attributes or functional negations: those
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index 1f594cac1..0f277146e 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -256,7 +256,7 @@
 $contextBytes = strlen(implode('', $contextTenAssets));
 // Baseline measured with folding disabled: 142,268 bytes for ten pages.
 $assert(1 === count($contextTwoAssets) && 1 === count($contextTenAssets), 'context-dependent arms fold without page copies', (string) json_encode(array_keys($contextTenAssets)));
-$assert($contextBytes - strlen(implode('', $contextTwoAssets)) < 6000, 'only per-page arm bytes grow, not the overlapping cascade suffix', (string) $contextBytes);
+$assert($contextBytes - strlen(implode('', $contextTwoAssets)) < 12000, 'per-page arm and no-route scope bytes grow, not the overlapping cascade suffix', (string) $contextBytes);
 $assert((bool) preg_match('/blocks-engine-control-[a-f0-9]{12}-[0-9]+/', implode('', $contextTenAssets)), 'fixture exercises positive identity-marker arms');
 foreach ($contextTen['pages'] as $planPage) {
     $document = new DOMDocument();
@@ -337,7 +337,7 @@
 }
 $group = ':where(:root:is(.' . $classes['b.html'] . ',.' . $classes['c.html'] . '))';
 $assert(str_contains($routeText, '.sample{'), 'plain variants inert on other routes remain shared');
-$assert(str_contains($routeText, 'html .sample') && str_contains($routeText, ':root .sample'), 'inert html and root arms retain their original compounds');
+$assert(str_contains($routeText, 'html:where(') && str_contains($routeText, ':root:where('), 'variant-only html and root arms merge their positive route scope');
 $droppedOverride = false;
 $conditionalImage = false;
 $transformer->visitStyleRules($routeText, static function (string $prelude, string $body, array $context) use (&$droppedOverride, &$conditionalImage, $classes): void {
@@ -348,7 +348,7 @@
 });
 $assert($droppedOverride, 'the inert dropped override retains its original conditional position');
 $assert($conditionalImage, 'page-only image rules retain their at-rule context');
-$assert(!str_contains($routeText, ':where(:root:is(.' . $classes['e.html'] . '))'), 'page-only image bridge inert elsewhere needs no positive route scope');
+$assert(str_contains($routeText, ':where(:root:is(.' . $classes['e.html'] . '))'), 'page-only image bridge excludes the unknown non-plan document');
 
 // Each route is compared with its independent public compilation, including
 // editor wrapper selectors. Apply exactly the root attributes the theme uses.
@@ -500,9 +500,9 @@
 // Rules that differ between pages only by page-local block markers. Every
 // page has its own buttons in a `.row-cols-3` grid, so `.row-cols-3>*` and
 // `.row>*` project onto each page's own per-element markers, which the page
-// projection packs as `>:is(:where(.m1),…)`. Those differences fold into one
-// unscoped union in the shared rule: the sheet grows by marker names only,
-// never by a route-scoped copy of the rule per page.
+// projection packs as `>:is(:where(.m1),…)`. With the virtual base route,
+// variant-only markers must now be scoped even if private to one plan page.
+// Small cases reconcile; larger ones may hit the unchanged byte-budget fallback.
 $markerCss = '.row{display:flex;flex-wrap:wrap}.row-cols-3>*{flex:0 0 auto;width:33.3%}.row>*{flex-shrink:0;max-width:100%}'
     . '.btn{background-color:#123456;color:#fff;border-radius:4px;padding:8px 16px}';
 for ($i = 0; $i < 120; ++$i) $markerCss .= '.marker-tail-' . $i . '{color:#123456;margin:0;padding:2px;border-top-width:1px}';
@@ -524,15 +524,17 @@
     $site = $markerSite($count);
     $text = implode('', $site['assets']);
     $markerSizes[$count] = strlen($text);
-    $assert(1 === count($site['assets']) && !str_contains(implode('', array_column($site['plan']['assets'], 'source_path')), '.page-'), $count . ' marker-only pages: one site-wide asset, no page copies', json_encode(array_keys($site['assets'])));
-    $assert(!str_contains($text, 'be-r-'), $count . ' marker-only pages: nothing is route scoped', $text);
-    $assert(array() === array_filter($site['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? '')), $count . ' marker-only pages: no route-scope fallback');
+    $fallbacks = array_filter($site['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? ''));
+    if (2 === $count) {
+        $assert(1 === count($site['assets']) && str_contains($text, 'be-r-') && !$fallbacks, 'small marker fixture scopes variants without copies');
+    } else {
+        $assert(count($site['assets']) > 1 && count($fallbacks) === 1, $count . ' marker pages: unknown non-plan markup retains the unchanged byte-budget fallback');
+    }
 }
 $markerText = implode('', $site['assets']);
-$assert((bool) preg_match('/:where\((?:\.blocks-engine-control-[a-f0-9]{12}-\d+,){3,}/', $markerText), 'page-local markers fold into one recompacted :where() list', $markerText);
-// Ten pages add only their marker names: under a fifth of the two-page
-// sheet. Route-scoped copies grew it by about 2.3 KB per page.
-$assert($markerSizes[10] < 1.2 * $markerSizes[2] && $markerSizes[10] - $markerSizes[6] < 1.2 * ($markerSizes[6] - $markerSizes[2]) + 512, 'site-wide sheet bytes stay roughly constant in the page count', json_encode($markerSizes));
+$assert((bool) preg_match('/:where\((?:\.blocks-engine-control-[a-f0-9]{12}-\d+,)+/', $markerText), 'same-route page-local markers still recompact into :where() lists');
+// Even the budget fallback must not duplicate the long unchanged tail.
+$assert($markerSizes[10] < 1.4 * $markerSizes[2] && $markerSizes[10] - $markerSizes[6] < 1.2 * ($markerSizes[6] - $markerSizes[2]) + 512, 'site-wide sheet bytes stay roughly constant in the page count', json_encode($markerSizes));
 $markerProbe = static function (array $plan, array $row): array {
     $assets = array_column($plan['assets'], null, 'target_path');
     $css = '';
@@ -551,17 +553,15 @@
     }
     return $styles;
 };
-// With every third page empty, the empty pages' projection is the shared
-// one: the other pages' marker rules are page-only. Each is inert off its
-// own page, so they join the sheet as unscoped unions; only rules whose arms
-// can match without a marker carry a route scope.
+// With every third page empty, its projection is the shared one. Other
+// pages' marker rules are page-only and must exclude unknown non-plan pages.
 $gapSite = $markerSite(9, true);
 $gapText = implode('', $gapSite['assets']);
 $scopedMarkerRule = false;
 $transformer->visitStyleRules($gapText, static function (string $prelude) use (&$scopedMarkerRule): void {
     if (str_contains($prelude, 'be-r-') && preg_match('/blocks-engine-control-[a-f0-9]{12}-\d+/', $prelude)) $scopedMarkerRule = true;
 });
-$assert(1 === count($gapSite['assets']) && !$scopedMarkerRule, 'page-only marker rules fold into unscoped unions', $gapText);
+$assert(1 === count($gapSite['assets']) && $scopedMarkerRule, 'page-only marker rules stay shared but exclude non-plan pages');
 foreach (array(array($site, false), array($gapSite, true)) as [$checked, $gaps]) {
     foreach ($checked['plan']['pages'] as $row) {
         $index = (int) substr($row['source_path'], 1);
@@ -574,7 +574,7 @@
 }
 echo 'Marker-only fixture author CSS: ' . json_encode($markerSizes) . " bytes for 2/6/10 pages\n";
 
-// Mixed real-site rule shapes: hundreds of safe marker arms must not inherit
+// Mixed real-site rule shapes: hundreds of base marker arms must not inherit
 // a shared-chrome arm's scope. Richtext swaps and a dropped plain rule use
 // the same per-arm membership path, including complementary scopes.
 $armSizes = array();
@@ -609,19 +609,19 @@
             $assert($expected->resolve($element, $properties, array()) === $actual->resolve($element, $properties, array()), 'mixed-arm computed-style parity ' . $count . ' routes: ' . $owner . ' ' . $element->textContent);
         }
     }
-    $assert(str_contains($text, ':root:not(') || 2 === $count, 'the dropped rule uses the shorter complementary scope');
+    $assert(str_contains($text, ':root:not(') || 2 === $count, 'the dropped base rule uses complementary scope');
 }
-$assert($armSizes[30] < 1.1 * $armSizes[2], 'mixed-arm site-wide bytes stay roughly constant in N', json_encode($armSizes));
+$assert($armSizes[30] < 1.4 * $armSizes[2], 'mixed-arm site-wide bytes stay roughly constant in N', json_encode($armSizes));
 
 // Private richtext carriers (including packed style$/editor-span variants)
-// can be emitted unscoped, but negations and unknown carriers cannot.
+// still need positive scope when absent from the virtual base projection.
 $richOne = 'blocks-engine-richtext-dddddddddddd-0';
 $richTwo = 'blocks-engine-richtext-eeeeeeeeeeee-0';
 $richList = static fn (string $marker): string => ':where(mark[style*="--blocks-engine-richtext-marker:' . $marker . ';"],mark[style$="--blocks-engine-richtext-marker:' . $marker . '"],span[data-blocks-engine-richtext-marker="' . $marker . '"])';
 $richPages = array('one' => array($richList($richOne) . '{color:red}'), 'two' => array($richList($richTwo) . '{color:red}'));
 $richMarkup = array('one' => '<mark style="--blocks-engine-richtext-marker:' . $richOne . ';">One</mark>', 'two' => '<span data-blocks-engine-richtext-marker="' . $richTwo . '">Two</span>');
 $privateRich = $transformer->scopeRouteVariants($richPages['one'], $richPages, array(), null, $richMarkup);
-$assert(!str_contains($privateRich['stylesheet'], 'be-r-'), 'private richtext marker swaps need no route scopes');
+$assert(str_contains($privateRich['stylesheet'], ':root:is('), 'variant-only private richtext excludes non-plan pages');
 $unknownRich = $transformer->scopeRouteVariants($richPages['one'], $richPages);
 $assert(str_contains($unknownRich['stylesheet'], 'be-r-'), 'missing carrier evidence never proves richtext arms inert');
 $negated = ':not(:where(.blocks-engine-control-aaaaaaaaaaaa-0)){color:red}';
@@ -697,12 +697,60 @@
 $assert($index->inertOutside('.absent:hover', array('plain-1' => true)), 'state-independent non-match is inert');
 $assert(!$index->inertOutside('.x:hover', array('plain-1' => true)), 'stateful structural match stays scoped');
 
+// Route state is restored after compilation. Neither DOM non-matches nor
+// private marker carriers can prove these arms inert on another route.
+$navMarkup = '<nav class="wp-block-navigation"><li class="wp-block-navigation-item"><a class="wp-block-navigation-item__content">Current</a></li></nav>';
+$navDocuments = array();
+foreach (array('kept', 'outside') as $owner) {
+    $dom = new DOMDocument();
+    @$dom->loadHTML('<html><body>' . $navMarkup . '</body></html>');
+    $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+    $navDocuments[$owner] = $dom;
+}
+$navIndex = new Automattic\BlocksEngine\PhpTransformer\Css\RouteSelectorMatchIndex($navDocuments);
+$navArm = ':root .wp-block-navigation .wp-block-navigation-item .wp-block-navigation-item__content[aria-current="page"]:not(.be-c-0):not(.be-c-0):not(.be-c-0):not(.be-c-0)';
+$navRule = $navArm . '{background:rgb(245,245,245);color:black}';
+$navResult = $transformer->scopeRouteVariants(array($navRule), array('kept' => array($navRule), 'outside' => array('')), array(), null, array('kept' => $navMarkup, 'outside' => $navMarkup), $navIndex);
+$assert(str_contains($navResult['stylesheet'], 'be-r-'), 'render-time aria-current rule stays route scoped');
+foreach ($navDocuments as $owner => $dom) {
+    $anchor = $dom->getElementsByTagName('a')->item(0);
+    $anchor->setAttribute('aria-current', 'page');
+    $expected = new StaticCssCascade($dom, 'kept' === $owner ? $navRule : '');
+    $actual = new StaticCssCascade($dom, $navResult['stylesheet']);
+    $assert($expected->resolve($anchor, array('background', 'color'), array()) === $actual->resolve($anchor, array('background', 'color'), array()), 'rendered current navigation parity: ' . $owner);
+}
+foreach (array('current-menu-item', 'current_page_item', 'current-menu-parent', 'current_page_parent', 'current-menu-ancestor', 'current_page_ancestor', 'blocks-engine-current-navigation-item', 'active', 'is-selected') as $stateClass) {
+    $assert(!$navIndex->inertOutside('.' . $stateClass . ' a', array('outside' => true)), 'render-time class cannot prove inertness: ' . $stateClass);
+}
+$stateMarker = 'blocks-engine-control-0123456789ab-99';
+$stateRule = ':where(.' . $stateMarker . ') [aria-current="page"]{color:red}';
+$stateResult = $transformer->scopeRouteVariants(array($stateRule), array('kept' => array($stateRule), 'outside' => array('')), array(), null, array('kept' => '<div class="' . $stateMarker . '"></div>', 'outside' => $navMarkup), $navIndex);
+$assert(str_contains($stateResult['stylesheet'], 'be-r-'), 'marker shortcut cannot bypass render-time route state');
+
+// A non-plan page has no route hook: its projection is the shared base, not
+// the union of page variants. A base arm must use exclusions even when a
+// positive list would be shorter. Variant-only arms must not leak either.
+$unplannedBase = 'body{font-family:Arial;line-height:1.5;color:red}';
+$unplannedPages = array('kept' => array($unplannedBase . 'body{background:blue}'));
+for ($k = 0; $k < 8; ++$k) $unplannedPages['dropped-' . $k] = array('');
+$unplannedResult = $transformer->scopeRouteVariants(array($unplannedBase), $unplannedPages);
+$assert(str_contains($unplannedResult['stylesheet'], ':root:not('), 'base rules exclude dropped plan routes rather than requiring a route class');
+foreach (array_merge(array(null), array_keys($unplannedPages)) as $owner) {
+    $dom = new DOMDocument();
+    @$dom->loadHTML('<html><body>Non-plan page</body></html>');
+    if (null !== $owner) $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+    $expected = new StaticCssCascade($dom, null === $owner ? $unplannedBase : implode('', $unplannedPages[$owner]));
+    $actual = new StaticCssCascade($dom, $unplannedResult['stylesheet']);
+    $body = $dom->getElementsByTagName('body')->item(0);
+    $assert($expected->resolve($body, array('font-family', 'line-height', 'color', 'background'), array()) === $actual->resolve($body, array('font-family', 'line-height', 'color', 'background'), array()), 'base/variant parity with route class: ' . ($owner ?? 'none'));
+}
+
 // Identical shared units align separately at each source occurrence.
 $marker = 'blocks-engine-control-0123456789ab-0';
 $unit = '.group>.btn{z-index:1}';
 $variant = '.group>.btn,:where(.' . $marker . '){z-index:1}';
 $duplicates = $transformer->scopeRouteVariants(array($unit . '.between{z-index:2}' . $unit), array('a' => array($variant . '.between{z-index:2}' . $variant)), array(), null, array('a' => '<p class="' . $marker . '">Probe</p>'));
-$assert(2 === substr_count($duplicates['stylesheet'], $marker), 'arm reconciliation rewrites every aligned shared unit without moving it');
+$assert(4 === substr_count($duplicates['stylesheet'], '{z-index:1}') && 4 === substr_count($duplicates['stylesheet'], $marker), 'arm reconciliation retains both occurrences with descendant/root route alternatives');
 
 // Identical leading imports remain site-wide despite non-dedupable units.
 $import = '@import url(/external/8e2e7efb6cf0362d/css-659beb8e13af.css);';

From 91ed1d394d806c0bdcfccc6abfadcb9714649810 Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Tue, 27 Oct 2026 10:52:40 -0400
Subject: [PATCH 09/10] Prove non-plan inertness from template-part engine
 classes

---
 .../src/ArtifactCompiler/ArtifactCompiler.php | 15 ++++--
 .../src/Css/CssStylesheetTransformer.php      | 40 ++++++++++++---
 .../shared-author-stylesheet-scaling.php      | 50 ++++++++++++++-----
 3 files changed, 82 insertions(+), 23 deletions(-)

diff --git a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
index f4b93d79d..bc142b679 100644
--- a/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
+++ b/php-transformer/src/ArtifactCompiler/ArtifactCompiler.php
@@ -381,8 +381,17 @@ private function finalizeArtifact(array $artifact, array $reduction): Transforme
         foreach ($compiledHtmlDocuments as $sourcePath => $document) {
             $pageMarkup[(string) ($ownedBy(array(array()), (string) $sourcePath)[0]['owner'] ?? '')] = $document['serialized_blocks'];
         }
+        // These are the template-part candidates consumed by compiledSiteReport
+        // and WordPressSitePlan's shell extraction. serialized_blocks alone omits
+        // extracted chrome. Use a conservative union of all candidate parts.
+        $sharedMarkup = '';
+        $parts = array_merge($this->compiledSiteTemplateParts($normalized['files']), $entryBlocks['shell_artifacts'], $inlineShellCompilation['artifacts']);
+        foreach ($compiledHtmlDocuments as $document) $parts = array_merge($parts, $document['shell_artifacts'] ?? array());
+        foreach ($parts as $part) $sharedMarkup .= "\n" . (string) ($part['block_markup'] ?? '');
+        foreach ($pageMarkup as &$markup) $markup .= "\n" . $sharedMarkup;
+        unset($markup);
         $authoredFiles = $normalized['files'];
-        $normalized['files'] = $this->applyAuthorStylesheetProjections($normalized['files'], $authorStylesheetProjections, $entryBlocks['author_stylesheet_projections'], (string) ($ownedBy(array(array()), $entryPath)[0]['owner'] ?? ''), $pageMarkup, $allDiagnostics);
+        $normalized['files'] = $this->applyAuthorStylesheetProjections($normalized['files'], $authorStylesheetProjections, $entryBlocks['author_stylesheet_projections'], (string) ($ownedBy(array(array()), $entryPath)[0]['owner'] ?? ''), $pageMarkup, $allDiagnostics, $sharedMarkup);
         foreach ($normalized['files'] as $document) {
             if ('html' !== ($document['kind'] ?? '') || !is_string($document['content'] ?? null) || !empty($document['metadata']['compilation']['included_component'])) continue;
             $documentFiles = $normalized['files'];
@@ -2519,7 +2528,7 @@ private function allocateStylesheetOccurrencePath(string $candidate, array &$res
      * @param string $primaryOwner The page that produced the primary projections, '' when they are site-wide.
      * @return array<int, array<string, mixed>>
      */
-    private function applyAuthorStylesheetProjections(array $files, array $projections, array $primaryProjections = array(), string $primaryOwner = '', array $pageMarkup = array(), array &$projectionDiagnostics = array()): array
+    private function applyAuthorStylesheetProjections(array $files, array $projections, array $primaryProjections = array(), string $primaryOwner = '', array $pageMarkup = array(), array &$projectionDiagnostics = array(), string $sharedMarkup = ''): array
     {
         $attributeStateMarkers = array();
         foreach ($projections as $projection) {
@@ -2684,7 +2693,7 @@ private function applyAuthorStylesheetProjections(array $files, array $projectio
             $sharedStylesheets = array($transformer->concatenateWithoutRedundantRules($sharedStylesheets));
             $fallbackSharedStylesheets = $sharedStylesheets;
             foreach ($pages as $owner => $_) $pages[$owner] = $pageProjections[$owner] ?? $pages[$owner];
-            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $pageProjections, $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup, $routeMatchIndex, $this->routeClasses);
+            $routed = $transformer->scopeRouteVariants($sharedStylesheets, $pageProjections, $routeIdentities, preg_match('/@namespace\b/i', implode('', $preambles)) ? '@namespace' : null, $pageMarkup, $routeMatchIndex, $this->routeClasses, $sharedMarkup);
             // Recompact only at serialization, merging compatible marker arms.
             $recompact = static fn (string $css): string => count($pageProjections) > 1 ? CssSelectorListCompactor::stylesheet($css, true) : $css;
             // A cached shared download replaces separate page-copy downloads.
diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index ece2798ac..7edcc09ab 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -256,6 +256,21 @@ private static function positiveMarker(string $arm): ?string
         return null;
     }
 
+    /** Required literal classes, never alternatives, negations or attributes. */
+    private static function requiredPageMarkers(string $arm): array
+    {
+        $markers = array();
+        if (str_contains($arm, '/*') || str_contains($arm, '\\')) return $markers;
+        $positive = self::positiveMarker($arm);
+        if (null !== $positive) $markers[$positive] = true;
+        preg_match_all('/\.(blocks-engine-[a-z-]+-[a-f0-9]{12}-[0-9]+)(?![a-zA-Z0-9_-])/', $arm, $matches, PREG_OFFSET_CAPTURE);
+        foreach ($matches[1] as [$marker, $offset]) {
+            $before = substr($arm, 0, $offset - 1);
+            if (0 === self::parenDepth($before) && substr_count($before, '[') === substr_count($before, ']')) $markers[$marker] = true;
+        }
+        return array_keys($markers);
+    }
+
     /**
      * Reconcile ordered page projections at the source cascade position.
      * Unsupported pages are returned separately for the legacy copy path.
@@ -278,14 +293,14 @@ private static function positiveMarker(string $arm): ?string
      * @param array<string,string> $routeIdentities Route identities by compilation owner.
      * @param array<string,string> $markup Canonical markup by compilation owner.
      * @param array<string,string> $routeClasses Site-wide collision-free classes by route identity.
+     * @param ?string $sharedMarkup Shared template-part markup; null means no carrier evidence.
      * `route_bytes` counts scope and split-rule overhead beyond the unscoped
      * arm union, plus scoped insertions, before final compaction.
      *
      * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>, route_bytes:int}
      */
-    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array(), ?RouteSelectorMatchIndex $matchIndex = null, array $routeClasses = array()): array
+    public function scopeRouteVariants(array $shared, array $pages, array $routeIdentities = array(), ?string $unsafeReason = null, array $markup = array(), ?RouteSelectorMatchIndex $matchIndex = null, array $routeClasses = array(), ?string $sharedMarkup = null): array
     {
-        $carriers = self::markerCarriers($markup);
         $base = array();
         foreach ($shared as $css) $this->collectRuleUnits($css, array(), $base);
         $base = self::withoutIgnoredImports($base);
@@ -383,11 +398,13 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         $routeClass = static fn ($owner): string => $routeClasses[$routeIdentities[$owner] ?? (string) $owner];
         $routeBytes = 0;
         $allRoutes = array_fill_keys(array_keys($pages), true);
-        // Non-plan WordPress pages receive the unprojected shared sheet. They
-        // have no route class and no compiled markup, so can never prove inertness.
+        // Non-plan content is unknown, but imported engine classes can only
+        // come from shared template parts. Never run a full matcher here.
         $baseOwner = '';
         while (isset($allRoutes[$baseOwner])) $baseOwner .= '#';
         $allRoutes[$baseOwner] = true;
+        if (null !== $sharedMarkup) $markup[$baseOwner] = $sharedMarkup;
+        $carriers = self::markerCarriers($markup);
         $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $baseOwner, $carriers, $markup, $matchIndex): void {
             [$lead, $rule] = self::splitLeadingComments($prototype['text']);
             $brace = (int) $this->nextRuleBoundary($rule, 0);
@@ -404,11 +421,18 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
             $groups = array();
             foreach ($members as $arm => $routes) {
                 $outside = array_diff_key($allRoutes, $routes);
-                $marker = self::positiveMarker($arm);
                 $inert = !$outside;
-                if (!$inert && !isset($outside[$baseOwner]) && !RouteSelectorMatchIndex::dependsOnRouteState($arm)) {
-                    $inert = null !== $marker && !array_diff_key($outside, $markup) && !array_diff_key($carriers[$marker] ?? array(), $routes);
-                    if (!$inert && null !== $matchIndex) $inert = $matchIndex->inertOutside($arm, $outside);
+                if (!$inert && !RouteSelectorMatchIndex::dependsOnRouteState($arm)) {
+                    $unproven = $outside;
+                    foreach (self::requiredPageMarkers($arm) as $marker) {
+                        // A missing required carrier disproves the arm on that
+                        // document, including required ancestors and subjects.
+                        foreach ($unproven as $owner => $_) {
+                            if (isset($markup[$owner]) && !isset($carriers[$marker][$owner])) unset($unproven[$owner]);
+                        }
+                    }
+                    $inert = !$unproven;
+                    if (!$inert && !isset($unproven[$baseOwner]) && null !== $matchIndex) $inert = $matchIndex->inertOutside($arm, $unproven);
                 }
                 $scope = '';
                 if (!$inert) {
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index 0f277146e..ce8191737 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -500,9 +500,8 @@
 // Rules that differ between pages only by page-local block markers. Every
 // page has its own buttons in a `.row-cols-3` grid, so `.row-cols-3>*` and
 // `.row>*` project onto each page's own per-element markers, which the page
-// projection packs as `>:is(:where(.m1),…)`. With the virtual base route,
-// variant-only markers must now be scoped even if private to one plan page.
-// Small cases reconcile; larger ones may hit the unchanged byte-budget fallback.
+// projection packs as `>:is(:where(.m1),…)`. Private markers are absent
+// from shared template parts, so even variant-only arms stay unscoped.
 $markerCss = '.row{display:flex;flex-wrap:wrap}.row-cols-3>*{flex:0 0 auto;width:33.3%}.row>*{flex-shrink:0;max-width:100%}'
     . '.btn{background-color:#123456;color:#fff;border-radius:4px;padding:8px 16px}';
 for ($i = 0; $i < 120; ++$i) $markerCss .= '.marker-tail-' . $i . '{color:#123456;margin:0;padding:2px;border-top-width:1px}';
@@ -525,16 +524,12 @@
     $text = implode('', $site['assets']);
     $markerSizes[$count] = strlen($text);
     $fallbacks = array_filter($site['diagnostics'], static fn (array $row): bool => 'author_stylesheet_route_scope_fallback' === ($row['code'] ?? ''));
-    if (2 === $count) {
-        $assert(1 === count($site['assets']) && str_contains($text, 'be-r-') && !$fallbacks, 'small marker fixture scopes variants without copies');
-    } else {
-        $assert(count($site['assets']) > 1 && count($fallbacks) === 1, $count . ' marker pages: unknown non-plan markup retains the unchanged byte-budget fallback');
-    }
+    $assert(1 === count($site['assets']) && !str_contains($text, 'be-r-') && !$fallbacks, $count . ' private marker pages: unscoped shared asset without fallback');
 }
 $markerText = implode('', $site['assets']);
 $assert((bool) preg_match('/:where\((?:\.blocks-engine-control-[a-f0-9]{12}-\d+,)+/', $markerText), 'same-route page-local markers still recompact into :where() lists');
-// Even the budget fallback must not duplicate the long unchanged tail.
-$assert($markerSizes[10] < 1.4 * $markerSizes[2] && $markerSizes[10] - $markerSizes[6] < 1.2 * ($markerSizes[6] - $markerSizes[2]) + 512, 'site-wide sheet bytes stay roughly constant in the page count', json_encode($markerSizes));
+// Growth is limited to marker names, not route scopes or cascade copies.
+$assert($markerSizes[10] < 1.2 * $markerSizes[2] && $markerSizes[10] - $markerSizes[6] < 1.2 * ($markerSizes[6] - $markerSizes[2]) + 512, 'site-wide sheet bytes stay roughly constant in the page count', json_encode($markerSizes));
 $markerProbe = static function (array $plan, array $row): array {
     $assets = array_column($plan['assets'], null, 'target_path');
     $css = '';
@@ -554,14 +549,14 @@
     return $styles;
 };
 // With every third page empty, its projection is the shared one. Other
-// pages' marker rules are page-only and must exclude unknown non-plan pages.
+// pages' marker rules are page-only but absent from shared chrome.
 $gapSite = $markerSite(9, true);
 $gapText = implode('', $gapSite['assets']);
 $scopedMarkerRule = false;
 $transformer->visitStyleRules($gapText, static function (string $prelude) use (&$scopedMarkerRule): void {
     if (str_contains($prelude, 'be-r-') && preg_match('/blocks-engine-control-[a-f0-9]{12}-\d+/', $prelude)) $scopedMarkerRule = true;
 });
-$assert(1 === count($gapSite['assets']) && $scopedMarkerRule, 'page-only marker rules stay shared but exclude non-plan pages');
+$assert(1 === count($gapSite['assets']) && !$scopedMarkerRule, 'page-only private marker rules stay shared and unscoped');
 foreach (array(array($site, false), array($gapSite, true)) as [$checked, $gaps]) {
     foreach ($checked['plan']['pages'] as $row) {
         $index = (int) substr($row['source_path'], 1);
@@ -697,6 +692,37 @@
 $assert($index->inertOutside('.absent:hover', array('plain-1' => true)), 'state-independent non-match is inert');
 $assert(!$index->inertOutside('.x:hover', array('plain-1' => true)), 'stateful structural match stays scoped');
 
+// Virtual-base proofs use required carriers only, not arbitrary DOM matches.
+// Exercise literal subjects, required ancestors, and the positiveMarker form.
+foreach (array('.%s', '.%s a', ':where(.%s)') as $shape) {
+    $sizes = array();
+    foreach (array(2, 12, 112) as $count) {
+        $pages = $markup = array();
+        for ($k = 0; $k < $count; ++$k) {
+            $marker = 'blocks-engine-control-' . substr(hash('sha256', (string) $k), 0, 12) . '-0';
+            $pages['route-' . $k] = array(sprintf($shape, $marker) . '{color:red}');
+            $markup['route-' . $k] = '<div class="' . $marker . '"><a>Local</a></div>';
+        }
+        $private = $transformer->scopeRouteVariants(array(), $pages, array(), null, $markup, null, array(), '<header>Shared</header>');
+        $assert(!str_contains($private['stylesheet'], 'be-r-') && 0 === $private['route_bytes'], $shape . ': N variant-only private classes stay unscoped: ' . $count);
+        // Serialized arm names grow; scope overhead stays constant at zero.
+        $sizes[$count] = $private['route_bytes'];
+        $header = '<header class="' . $marker . '"><a>Shared</a></header>';
+        $carried = $transformer->scopeRouteVariants(array(), $pages, array(), null, $markup, null, array(), $header);
+        $assert(str_contains($carried['stylesheet'], 'be-r-'), $shape . ': shared-header carrier prevents base inertness: ' . $count);
+        $dom = new DOMDocument();
+        @$dom->loadHTML('<html><body>' . $header . '</body></html>');
+        $cascade = new StaticCssCascade($dom, $carried['stylesheet']);
+        $target = str_ends_with($shape, ' a') ? $dom->getElementsByTagName('a')->item(0) : $dom->getElementsByTagName('header')->item(0);
+        $assert(!isset($cascade->resolve($target, array('color'), array())['color']), 'variant does not paint shared header on a no-route page');
+    }
+    $assert(1 === count(array_unique($sizes)), 'private marker scope overhead is constant in N');
+}
+foreach (array(':not(.blocks-engine-control-aaaaaaaaaaaa-0)', ':is(.blocks-engine-control-aaaaaaaaaaaa-0,a)', '[title=".blocks-engine-control-aaaaaaaaaaaa-0"]') as $arm) {
+    $result = $transformer->scopeRouteVariants(array(), array('one' => array($arm . '{color:red}')), array(), null, array('one' => ''), null, array(), '');
+    $assert(str_contains($result['stylesheet'], 'be-r-'), 'optional or quoted markers do not prove base inertness: ' . $arm);
+}
+
 // Route state is restored after compilation. Neither DOM non-matches nor
 // private marker carriers can prove these arms inert on another route.
 $navMarkup = '<nav class="wp-block-navigation"><li class="wp-block-navigation-item"><a class="wp-block-navigation-item__content">Current</a></li></nav>';

From b960d39e6d1e9a486ce185b47710e3df7597d1a3 Mon Sep 17 00:00:00 2001
From: Chris Huber <chris.huber@automattic.com>
Date: Tue, 27 Oct 2026 12:55:59 -0400
Subject: [PATCH 10/10] Use the shortest base-excluding route scope and budget
 shipped bytes

---
 .../src/Css/CssStylesheetTransformer.php      | 42 ++++++++++-------
 .../WordPressSitePlan/DocumentRootContext.php |  2 +-
 .../shared-author-stylesheet-scaling.php      | 47 ++++++++++++++++---
 .../unit/document-root-font-ownership.php     |  2 +-
 4 files changed, 67 insertions(+), 26 deletions(-)

diff --git a/php-transformer/src/Css/CssStylesheetTransformer.php b/php-transformer/src/Css/CssStylesheetTransformer.php
index 7edcc09ab..f758eb258 100644
--- a/php-transformer/src/Css/CssStylesheetTransformer.php
+++ b/php-transformer/src/Css/CssStylesheetTransformer.php
@@ -286,16 +286,16 @@ private static function requiredPageMarkers(string $arm): array
      * @param array<string,list<string>> $pages
      * Aligned rules are expanded into arms, each with its own route set.
      * Arms proven inert outside that set stay unscoped; all others use the
-     * positive scope for variant-only arms, or a complementary negative scope
-     * for arms retained by the virtual no-route-class base document. Only validated lists
+     * shortest base-excluding scope for variant-only arms, or a complementary
+     * negative scope for arms retained by the virtual base document. Only validated lists
      * are split, preserving invalid-selector isolation and source order.
      *
      * @param array<string,string> $routeIdentities Route identities by compilation owner.
      * @param array<string,string> $markup Canonical markup by compilation owner.
      * @param array<string,string> $routeClasses Site-wide collision-free classes by route identity.
      * @param ?string $sharedMarkup Shared template-part markup; null means no carrier evidence.
-     * `route_bytes` counts scope and split-rule overhead beyond the unscoped
-     * arm union, plus scoped insertions, before final compaction.
+     * `route_bytes` counts shipped growth after serialization and recompaction
+     * over the unscoped arm-union sheet (including inserted arms).
      *
      * @return array{stylesheet:string, scoped:list<string>, reasons:array<string,string>, route_bytes:int}
      */
@@ -396,7 +396,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         foreach (array_keys($pages) as $owner) $identities[] = $routeIdentities[$owner] ?? (string) $owner;
         $routeClasses = $routeClasses ?: \Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClasses($identities);
         $routeClass = static fn ($owner): string => $routeClasses[$routeIdentities[$owner] ?? (string) $owner];
-        $routeBytes = 0;
+        $unionOutput = array();
         $allRoutes = array_fill_keys(array_keys($pages), true);
         // Non-plan content is unknown, but imported engine classes can only
         // come from shared template parts. Never run a full matcher here.
@@ -405,7 +405,7 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
         $allRoutes[$baseOwner] = true;
         if (null !== $sharedMarkup) $markup[$baseOwner] = $sharedMarkup;
         $carriers = self::markerCarriers($markup);
-        $emitArms = function (array $units, array $prototype, bool $insertion = false) use (&$output, &$routeBytes, $routeClass, $allRoutes, $baseOwner, $carriers, $markup, $matchIndex): void {
+        $emitArms = function (array $units, array $prototype) use (&$output, &$unionOutput, $routeClass, $allRoutes, $baseOwner, $carriers, $markup, $matchIndex): void {
             [$lead, $rule] = self::splitLeadingComments($prototype['text']);
             $brace = (int) $this->nextRuleBoundary($rule, 0);
             $body = substr($rule, $brace);
@@ -439,13 +439,19 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                     if (isset($routes[$baseOwner])) {
                         $scope = ':where(:root:not(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($outside))) . '))';
                     } else {
-                        $scope = ':where(:root:is(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($routes))) . '))';
+                        $positive = ':where(:root:is(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($routes))) . '))';
+                        unset($outside[$baseOwner]);
+                        // The exact marker excludes non-plan documents even when
+                        // every plan route keeps this variant-only arm.
+                        $negative = ':where(:root.be-r' . ($outside ? ':not(' . implode(',', array_map(static fn ($owner): string => '.' . $routeClass($owner), array_keys($outside))) . ')' : '') . ')';
+                        $scope = strlen($positive) <= strlen($negative) ? $positive : $negative;
                     }
                 }
                 $groups[$scope][] = $arm;
             }
-            $emittedBytes = 0;
-            $unionBytes = strlen($lead . implode(',', array_keys($members)) . $body);
+            $union = $prototype;
+            $union['text'] = $lead . implode(',', array_keys($members)) . $body;
+            $unionOutput[] = $union;
             foreach ($groups as $scope => $arms) {
                 $unit = $prototype;
                 if ('' !== $scope) {
@@ -454,16 +460,11 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                 }
                 $unit['text'] = $lead . implode(',', $arms) . $body;
                 $output[] = $unit;
-                $emittedBytes += strlen($unit['text']);
-                // A scoped insertion is entirely new shared-download cost,
-                // not just the prefix added to its unscoped selector list.
-                if ($insertion && '' !== $scope) $routeBytes += strlen(implode(',', $groups[$scope]) . $body);
                 $lead = '';
             }
-            $routeBytes += max(0, $emittedBytes - $unionBytes);
         };
         for ($i = 0; $i <= count($base); ++$i) {
-            foreach ($siteInsertions[$i] ?? array() as $unit) $output[] = $unit;
+            foreach ($siteInsertions[$i] ?? array() as $unit) { $output[] = $unit; $unionOutput[] = $unit; }
             $groups = array();
             foreach ($insertions[$i] ?? array() as $owner => $units) foreach ($units as $ordinal => $unit) {
                 $groupKey = serialize(array($ordinal, $key($unit)));
@@ -472,14 +473,19 @@ public function scopeRouteVariants(array $shared, array $pages, array $routeIden
                 $groups[$groupKey]['routes'][$owner] = $unit;
             }
             uasort($groups, static fn (array $a, array $b): int => $a['ordinal'] <=> $b['ordinal']);
-            foreach ($groups as $group) $emitArms($group['routes'], $group['unit'], true);
+            foreach ($groups as $group) $emitArms($group['routes'], $group['unit']);
             if ($i === count($base)) break;
-            if (empty($changes[$i])) { $output[] = $base[$i]; continue; }
+            if (empty($changes[$i])) { $output[] = $base[$i]; $unionOutput[] = $base[$i]; continue; }
             $units = array();
             foreach ($allRoutes as $owner => $_) $units[$owner] = array_key_exists($owner, $changes[$i]) ? $changes[$i][$owner] : $base[$i];
             $emitArms($units, $base[$i]);
         }
-        return array('stylesheet' => $this->serializeRuleUnits($output), 'scoped' => $scoped, 'reasons' => $reasons, 'route_bytes' => $routeBytes);
+        $stylesheet = $this->serializeRuleUnits($output);
+        // Match ArtifactCompiler's final serialization, not the expanded arms:
+        // identical scope prefixes can ship once for an entire selector group.
+        $recompact = static fn (string $css): string => count($pages) > 1 ? CssSelectorListCompactor::stylesheet($css, true) : $css;
+        $routeBytes = max(0, strlen($recompact($stylesheet)) - strlen($recompact($this->serializeRuleUnits($unionOutput))));
+        return array('stylesheet' => $stylesheet, 'scoped' => $scoped, 'reasons' => $reasons, 'route_bytes' => $routeBytes);
     }
 
     /**
diff --git a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
index 292d54933..9b229e532 100644
--- a/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
+++ b/php-transformer/src/WordPressSitePlan/DocumentRootContext.php
@@ -20,7 +20,7 @@ public static function fromHtml(string $html, string $tagName = 'html'): array
     public static function metadataFromHtml(string $html, string $routeIdentity = '', ?string $routeClass = null): array
     {
         $attributes = DocumentRootAttributes::forDocument($html);
-        if ('' !== $routeIdentity) $attributes['html']['class'] = trim(($attributes['html']['class'] ?? '') . ' ' . ($routeClass ?? self::routeClass($routeIdentity)));
+        if ('' !== $routeIdentity) $attributes['html']['class'] = trim(($attributes['html']['class'] ?? '') . ' be-r ' . ($routeClass ?? self::routeClass($routeIdentity)));
         return array('root_attributes' => $attributes['html'], 'body_attributes' => $attributes['body']);
     }
 
diff --git a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
index ce8191737..9a0666663 100644
--- a/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
+++ b/php-transformer/tests/contract/shared-author-stylesheet-scaling.php
@@ -331,7 +331,8 @@
 foreach ($routePlan['pages'] as $row) {
     $class = Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass(WordPressSitePlan::identity('page', $row['source_path'], $row['source_path']));
     $classes[$row['source_path']] = $class;
-    $assert(in_array($class, explode(' ', $row['document_metadata']['root_attributes']['class'] ?? ''), true), 'document root carries its source route class: ' . $row['source_path']);
+    $rootClasses = explode(' ', $row['document_metadata']['root_attributes']['class'] ?? '');
+    $assert(in_array($class, $rootClasses, true) && in_array('be-r', $rootClasses, true), 'every plan document root carries exact plan marker and source route class: ' . $row['source_path']);
     $functions = array_column($routePlan['writes'], null, 'target_path')['functions.php']['payload']['data'];
     $assert(str_contains($functions, $class) && str_contains($functions, 'blocksEngineDocumentContext') && str_contains($functions, 'language_attributes'), 'the same root registry reaches front end and editor: ' . $row['source_path']);
 }
@@ -596,7 +597,7 @@
     foreach ($pages as $owner => $sheets) {
         $dom = new DOMDocument();
         @$dom->loadHTML('<html><body>' . $markup[$owner] . '</body></html>');
-        $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+        $dom->documentElement->setAttribute('class', 'be-r ' . Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
         $expected = new StaticCssCascade($dom, implode('', $sheets));
         $actual = new StaticCssCascade($dom, $text);
         foreach (array('p', 'mark') as $tag) foreach ($dom->getElementsByTagName($tag) as $element) {
@@ -639,7 +640,7 @@
         $plainMarkup[$owner] = '<div id="page-only-' . $k . '"><p class="x native">Probe</p></div>';
         $dom = new DOMDocument();
         @$dom->loadHTML('<html><body>' . $plainMarkup[$owner] . '</body></html>');
-        $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+        $dom->documentElement->setAttribute('class', 'be-r ' . Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
         $plainDocuments[$owner] = $dom;
     }
     $index = new Automattic\BlocksEngine\PhpTransformer\Css\RouteSelectorMatchIndex($plainDocuments);
@@ -720,7 +721,7 @@
 }
 foreach (array(':not(.blocks-engine-control-aaaaaaaaaaaa-0)', ':is(.blocks-engine-control-aaaaaaaaaaaa-0,a)', '[title=".blocks-engine-control-aaaaaaaaaaaa-0"]') as $arm) {
     $result = $transformer->scopeRouteVariants(array(), array('one' => array($arm . '{color:red}')), array(), null, array('one' => ''), null, array(), '');
-    $assert(str_contains($result['stylesheet'], 'be-r-'), 'optional or quoted markers do not prove base inertness: ' . $arm);
+    $assert(str_contains($result['stylesheet'], ':root.be-r'), 'optional or quoted markers do not prove base inertness: ' . $arm);
 }
 
 // Route state is restored after compilation. Neither DOM non-matches nor
@@ -730,7 +731,7 @@
 foreach (array('kept', 'outside') as $owner) {
     $dom = new DOMDocument();
     @$dom->loadHTML('<html><body>' . $navMarkup . '</body></html>');
-    $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+    $dom->documentElement->setAttribute('class', 'be-r ' . Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
     $navDocuments[$owner] = $dom;
 }
 $navIndex = new Automattic\BlocksEngine\PhpTransformer\Css\RouteSelectorMatchIndex($navDocuments);
@@ -764,7 +765,7 @@
 foreach (array_merge(array(null), array_keys($unplannedPages)) as $owner) {
     $dom = new DOMDocument();
     @$dom->loadHTML('<html><body>Non-plan page</body></html>');
-    if (null !== $owner) $dom->documentElement->setAttribute('class', Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
+    if (null !== $owner) $dom->documentElement->setAttribute('class', 'be-r ' . Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::routeClass($owner));
     $expected = new StaticCssCascade($dom, null === $owner ? $unplannedBase : implode('', $unplannedPages[$owner]));
     $actual = new StaticCssCascade($dom, $unplannedResult['stylesheet']);
     $body = $dom->getElementsByTagName('body')->item(0);
@@ -810,6 +811,40 @@
 $assert(3 === count(array_unique($collisionClasses)) && 14 === strlen($collisionClasses['collision-55045']) && 13 === strlen($collisionClasses['ordinary-route']), 'colliding hashes lengthen only as needed');
 $assert($collisionClasses === $rootContext::routeClasses(array_reverse($collisionIdentities)), 'class allocation is independent of route order');
 
+// A majority variant uses exact plan membership plus short exclusions.
+$majorityPages = array();
+for ($k = 0; $k < 32; ++$k) $majorityPages['majority-' . $k] = array($k ? '.majority{color:red}' : '');
+$majority = $transformer->scopeRouteVariants(array(), $majorityPages);
+$assert(str_contains($majority['stylesheet'], ':root.be-r:not('), 'majority variant chooses short exact exclusion scope');
+foreach (array_merge(array(null, 'pretend'), array_keys($majorityPages)) as $owner) {
+    $dom = new DOMDocument();
+    @$dom->loadHTML('<html><body><p class="majority">Probe</p></body></html>');
+    if (null !== $owner) {
+        $metadata = Automattic\BlocksEngine\PhpTransformer\WordPressSitePlan\DocumentRootContext::metadataFromHtml('', $owner);
+        $dom->documentElement->setAttribute('class', 'pretend' === $owner ? 'be-r-impostor' : $metadata['root_attributes']['class']);
+    }
+    $cascade = new StaticCssCascade($dom, $majority['stylesheet']);
+    $painted = isset($cascade->resolve($dom->getElementsByTagName('p')->item(0), array('color'), array())['color']);
+    $assert($painted === (null !== $owner && 'pretend' !== $owner && 'majority-0' !== $owner), 'majority exclusion parity: ' . ($owner ?? 'no route'));
+}
+
+// Many arms share one scope: expanded accounting exceeds the budget, but
+// the serialized common prefix ships once and must not trigger page copies.
+$groupArms = array();
+for ($k = 0; $k < 400; ++$k) $groupArms[] = 'p.a' . $k;
+$groupRule = implode(',', $groupArms) . '{color:red}';
+$groupPages = array('kept' => array($groupRule), 'dropped' => array(''));
+$grouped = $transformer->scopeRouteVariants(array($groupRule), $groupPages);
+$compact = static fn (string $css): string => Automattic\BlocksEngine\PhpTransformer\Css\CssSelectorListCompactor::stylesheet($css, true);
+$shippedGrowth = strlen($compact($grouped['stylesheet'])) - strlen($compact($groupRule));
+$groupBudget = max(8192, 3 * strlen($groupRule));
+$assert($grouped['route_bytes'] === $shippedGrowth && $shippedGrowth < $groupBudget && strlen($grouped['stylesheet']) - strlen($groupRule) > $groupBudget, 'budget counts shipped common scope once, not per expanded arm');
+$groupDiagnostics = array();
+$method = new ReflectionMethod(ArtifactCompiler::class, 'applyAuthorStylesheetProjections');
+$groupFiles = array(array('path' => 'group.css', 'kind' => 'css', 'content' => $groupRule));
+$groupOutput = $method->invokeArgs(new ArtifactCompiler(), array($groupFiles, array(array('path' => 'group.css', 'content' => $groupRule, 'owner' => 'kept'), array('path' => 'group.css', 'content' => '', 'owner' => 'dropped')), array(array('path' => 'group.css', 'content' => $groupRule)), '', array(), &$groupDiagnostics));
+$assert(!$groupDiagnostics && !str_contains(implode('', array_column($groupOutput, 'path')), '.page-'), 'compiler accepts grouped shipped scope below budget');
+
 // Exercise the compiler budget with explicit projections: all routes carry
 // the subject, so dropping a rule cannot be proven inert. Long unchanged
 // cascade tails make legacy copies costly in aggregate, not individually.
diff --git a/php-transformer/tests/unit/document-root-font-ownership.php b/php-transformer/tests/unit/document-root-font-ownership.php
index 69765057a..93efe7b91 100644
--- a/php-transformer/tests/unit/document-root-font-ownership.php
+++ b/php-transformer/tests/unit/document-root-font-ownership.php
@@ -23,7 +23,7 @@
 $assert('document' === ($pages['index.html']['document_metadata']['root_attributes']['id'] ?? null), 'root ID travels alongside classes');
 $bootstrap = '';
 foreach ($plan['writes'] as $write) if ('functions.php' === $write['target_path']) $bootstrap = $write['payload']['data'];
-$assert(str_contains($bootstrap, "'language_attributes'") && str_contains($bootstrap, 'WP_HTML_Tag_Processor') && str_contains($bootstrap, 'type-a be-r-') && str_contains($bootstrap, 'type-b be-r-'), 'generated runtime adopts route-owned root identity through native HTML API');
+$assert(str_contains($bootstrap, "'language_attributes'") && str_contains($bootstrap, 'WP_HTML_Tag_Processor') && str_contains($bootstrap, 'type-a be-r be-r-') && str_contains($bootstrap, 'type-b be-r be-r-'), 'generated runtime adopts route-owned root identity through native HTML API');
 $assert(str_contains($bootstrap, "'_blocks_engine_reconciliation_identity'") && str_contains($bootstrap, 'is_front_page()'), 'runtime selection uses destination identity with route fallback');
 $css = implode("\n", array_column(array_filter($plan['assets'], static fn(array $asset): bool => 'css' === $asset['kind']), 'content'));
 $assert(str_contains($css, '.type-a') && str_contains($css, '.type-b') && str_contains($css, '@media'), 'authored root selector and conditional cascade stay intact');