[selectors-4][editorial] First, and incomplete, cut at an Explainer, for eventual TAG review #12799

Author: svgeesus

selectors-4/Selectors _4_Explainer.md

@@ -1,10 +1,11 @@
 # Selectors 4 Explainer
 
-## Authors
+## Author
 
  - Chris Lilley (W3C)
 
 ## Participate
+
 - [Issue tracker](https://github.com/w3c/csswg-drafts/issues?q=state%3Aopen%20label%3A%22selectors-4%22)
 
 ## What are Selectors
@@ -20,7 +21,10 @@ and in [Selectors 3](https://www.w3.org/TR/selectors-3/)
 (W3C Recommendation, 2018).
 
 [Selectors 4](https://www.w3.org/TR/selectors-4/)
-is now widely deployed, stable,
+is an incremental improvement to previous Selectors specifications,
+responding to common user needs.
+
+It is now widely deployed, stable,
 and is being prepared for W3C Recommendation,
 while less-mature Selectors have been moved to
 [Selectors 5](https://www.w3.org/TR/selectors-5/).
@@ -52,9 +56,20 @@ These are both majority uses, hence the naming of the specification.
     formerly called `:matches()`,
     has been added to allow stylesheet authors to group functionally related elements,
     such as any element that is either hovered or focussed.
+- The specificity-adjusting [`:where()`](https://www.w3.org/TR/selectors-4/#zero-matches) pseudo-class
+    has been added to construct zero-specificity filters,
+    which are easy to override,
+    replacing the many ugly hacks previously required to simulate this
+- The relational [`:has()`]() pseudo-class
 
 ### Interactivity and Forms
 
+- Added the [`:modal`](https://www.w3.org/TR/selectors-4/#modal-state) pseudo-class,
+    to select elements which exclude interaction with all other elements until dismissed
+- Added [`:open`](https://www.w3.org/TR/selectors-4/#open-state)
+    and [`:popover-open`](https://www.w3.org/TR/selectors-4/#popover-open-state) pseudo-classes,
+    primarily but not exclusively to use with HTML forms
+
 ### Hyperlinking Support
 
 - Added [`:any-link`](https://www.w3.org/TR/selectors-4/#the-any-link-pseudo) to match
@@ -66,17 +81,34 @@ These are both majority uses, hence the naming of the specification.
     and negation [`:not()`](https://www.w3.org/TR/selectors-4/#negation) pseudo-classes
     to accept a **selector list**.
 
+### Media State
+
+- Added the `:playing`, `:paused` and `:seeking`  [media playback state](https://www.w3.org/TR/selectors-4/#video-state) pseudo classes,
+    to enable selecting on the dynamic state of video elements,
+    functionality which previously required JavaScript
+- Added the `:buffered` and `:stalled`
+    [media loading state](https://www.w3.org/TR/selectors-4/#media-loading-state) pseudo-classes,
+    again providing functionality that previously needed JavaScript
+- Added the [`:fullscreen`](https://www.w3.org/TR/selectors-4/#fullscreen-state) 
+    and [`:picture-in-picture`](https://www.w3.org/TR/selectors-4/#pip-state) pseudo-classes
+- Added the `:muted` and `volume-locked` pseudo-classes
+    for sound-producing elements
+
 ### Internationalization Improvements
 
-- Expanded the content language [`:lang()`](https://www.w3.org/TR/selectors-4/#the-lang-pseudo) pseudo-class
-    to accept **wildcard matching** and **lists of language codes**.
-- 
+- Expanded the content language 
+    [`:lang()`](https://www.w3.org/TR/selectors-4/#the-lang-pseudo) pseudo-class
+    to accept **wildcard matching** and **lists of language codes**
+    for greater flexibility styling related languages
+- Added the content language writing direction
+    [`dir()`](https://www.w3.org/TR/selectors-4/#the-dir-pseudo) pseudo-class
+    for convenient styling of left-to-right and righ-to-left content
+- Clarified case-insensitive string matching
 
 ### Consolidation
 
 Selectors first introduced in
 [CSS Basic UI 3](https://www.w3.org/TR/css-ui-3/),
-such as 
 have been moved to the main Selectors 4 specification.
 
 ## Further reading