[css-page-3] Proposed spec changes for safe printable insets. #11395 (#13190)

* Proposed spec changes for safe printable insets.

See
#11395 (comment)

* Update Overview.bs

Issue for multiple at-rules not supported by bikeshed.

Author: mstensho

css-page-3/Overview.bs

@@ -1258,7 +1258,7 @@ properties:
 		is larger than the height of the top page margin,
 		then any ''margin-top/auto'' values for 'margin-top' or 'margin-bottom' are,
 		for the following rules,
-		treated as zero.
+		treated as <<safe-printable-inset>>.
 
 	<li>
 		If at this point all of 'height', 'margin-top', and 'margin-bottom'
@@ -1271,14 +1271,20 @@ properties:
 		follows from the equality.
 
 	<li>
-		If 'height' is set to ''height/auto'', any other <css>auto</css> values become ''0'' and
-		'height' follows from the resulting equality
+		If 'height' is set to ''height/auto'', any <css>auto</css> margin
+		values become <<safe-printable-inset>> and 'height' follows from the
+		resulting equality.
 
 	<li>
 		If both 'margin-top' and 'margin-bottom' are ''margin-top/auto'',
-		their used values are equal.
+		their used values become equal.
 		This vertically centers the page-margin box content
 		within the top page margin.
+		If <<safe-printable-inset>> is not ''0'' here, an additional adjustment step
+		is performed: If the resulting 'margin-top' in the previous step became
+		less than <<safe-printable-inset>>, subtract as much as possible of this
+		difference from 'margin-bottom' without making it negative. Then add
+		this value to 'margin-top'.
 </ol>
 
 	The same rules apply to the bottom page-margin boxes (bottom-left-corner,
@@ -2060,6 +2066,73 @@ Positioning the page box on the sheet</h3>
 	The user agent may wish to consult the user in this
 	regard.<!-- "Page selector and page context" -->
 
+<h3 id="page-margin-safety">
+Staying within the printable area: the '@page/page-margin-safety' property</h3>
+
+	<pre class='descdef'>
+	Name: page-margin-safety
+	For: @page
+	Value: none | clamp | add
+	Initial: auto
+	Computed Value: as specified
+	</pre>
+
+	<div class=issue>
+		<p>
+			The "For:" field above should also include the at-rules
+			@top-left-corner, @top-left, @top-center, @top-right,
+			@top-right-corner, @right-top, @right-middle,
+			@right-bottom, @bottom-right-corner, @bottom-right,
+			@bottom-center, @bottom-left, @bottom-left-corner,
+			@left-bottom, @left-middle, @left-top.
+		</p>
+		<p>However, multiple at-rules here are currently not supported by bikeshed.</p>
+	</div>
+
+	Most printers have a small region along each edge of the page sheet
+	which is unprintable, typically due to the printer's paper handling
+	mechanism. This property can be used to help staying within the
+	<a>printable area</a>, by adjusting the page margins and page margin box
+	margins.
+
+	Some printers don't have a uniform unprintable area width along each of
+	the four paper edges, and the printer may rotate the print output at
+	their own discretion. The user agent may therefore not be able to make
+	assumptions about which edge will be fed first into the printer, or what
+	orientation the sheet of paper has. If the user agent cannot make such
+	assumptions, only one will be provided (to be used on all 4 sides): The
+	larger of these four values. Otherwise, if the user agent can trust that
+	the four values are usable individually, and that no rotation is going
+	to take place, and so on, each side of the page may have individual
+	values.
+
+	Let this value be <dfn><<safe-printable-inset>></dfn>, which is a <<length>>.
+
+	This property only affects margins that are adjacent to one of the edges
+	of a sheet. For all other margins, i.e. those margins facing the
+	page's <a>page area</a>, its value is 0. In other words, when
+	page-margin-safety is specified in a margin context, it will only have
+	an effect at one or two sides. For instance, @page { @top-right-corner {
+	page-margin-safety: clamp; } } may only clamp top and right margin
+	values, as the bottom and left margins face the document's contents, not
+	the page's edges.
+
+	<dl dfn-for="@page/page-margin-safety" dfn-type=value>
+		<dt><dfn>none</dfn></dt>
+		<dd>Margins are not affected by unprintable regions</dd>
+		<dt><dfn>clamp</dfn></dt>
+		<dd>
+			The used margin values will be the maximum of the
+			resolved value, and <<safe-printable-inset>> for the
+			given side.
+		</dd>
+		<dt><dfn>add</dfn></dt>
+		<dd>
+			The used margin values will be the resolved value, plus
+			<<safe-printable-inset>> for the given side.
+		</dd>
+	</dl>
+
 <h2 id="page-breaks">
 		<span id="pg-br-before-after"></span>
 		<span id="page-break-before"></span>