[CSS-Pseudo] Correct custom properties in highlights (#11528)

It was resolved in #11317, CSS Highlight Pseudos should allow custom property definitions within the rule itself, but take any missing custom properties from the originating element.

This PR updates the spec to re-list custom properties as supported on highlights. Furthermore, it fixes the description of custom property inheritance that was wrong even for the prior version of the spec.

css-pseudo-4/Overview.bs

@@ -910,6 +910,8 @@ Styling Highlights</h3>
     <wpt>
     textpath-selection-011.html
     </wpt>
+
+    <li>[=custom properties=] [[!CSS-VARIABLES-1]]
   </ul>
 
   Issue: Are there any other properties that should be included here?
@@ -1106,15 +1108,14 @@ Cascading and Per-Element Highlight Styles</h3>
   its [=specified value=] is determined by inheritance from
   the corresponding <a>highlight pseudo-element</a>
   of its <a>originating element</a>'s parent element.
-  This occurs regardless of whether that property is an <a>inherited property</a>
-  (and regardless of whether that property is a [=custom property=]
-  that is registered to [=CSS/inherit=] or not).
+  This occurs regardless of whether that property is an <a>inherited property</a>.
 
-  Additionally, for [=highlight pseudo-elements=] originating from the [=root element=]:
-  <ul>
-   <li> the inherited value of 'color' is ''currentColor'', not the [=initial value=].
-   <li> [=custom properties=] inherit from the [=root element=].
-  </ul>
+  Additionally, for [=highlight pseudo-elements=] originating from the [=root element=]
+  the inherited value of 'color' is ''currentColor'', not the [=initial value=].
+
+  All [=custom properties=] inherit from the <a>originating element</a>,
+  regardless of whether that property is a [=custom property=]
+  that is registered to [=CSS/inherit=] or not.
 
   <wpt>
   active-selection-051.html
@@ -1201,18 +1202,21 @@ Cascading and Per-Element Highlight Styles</h3>
         text-decoration-color: var(--decoration-color, red);
       }
     </pre>
-    A div's selection highlight would be a green background to the
-    selected content, with a 1px thick blue underline. The
-    ''--background-color'' custom property is not found on the
-    ''div::selection'' nor ''::selection'' so it is taken from the
-    ''::root'' custom property match. The ''--decoration-thickness''
-    property is inherited from ''::selection'' via the highlight
-    cascade and ''--decoration-color'' is found in
-    ''div::selection'' itself.
+    The universal ''::selection'' uses the user-agent's default styling
+    because it only defines custom properties,
+    with no properties that influence the appearance.
+    A div's selection highlight would apply a green background to the selected content,
+    with a 10px thick blue underline.
+    Since ''--background-color'' and ''--decoration-thickness'' custom properties
+    are not specified on the ''div::selection'' peudo-element,
+    they are inherited from its originating <{div}> element,
+    which itself inherits the custom properties from the root.
+    However since ''--decoration-color'' is specified on the ''div::selection'' itself,
+    its value from the [=originating element=] is not used.
   </div>
 
-  Note: This behavior is intended to accomodate the standard practice of
-  defining document-wide custom properties on '':root''.
+  Note: This behavior allows control of selection with custom properties
+  in a way that is compatible with pre-existing implementations.
 
 <h3 id="highlight-painting">
 Painting the Highlight</h3>
@@ -1909,7 +1913,8 @@ Changes</h2>
       application and inheritance model for [=highlight pseudo-elements=]
       (Issues <a href="https://github.com/w3c/csswg-drafts/issues/7591">7591</a>,
        <a href="https://github.com/w3c/csswg-drafts/issues/7580">7580</a>,
-       <a href="https://github.com/w3c/csswg-drafts/issues/6641">6641</a>)
+       <a href="https://github.com/w3c/csswg-drafts/issues/6641">6641</a>,
+       <a href="https://github.com/w3c/csswg-drafts/issues/11317">11317</a>)
     <li>Defined ''::highlight()'' and improved cross-referencing to [[CSS-HIGHLIGHT-API-1]].
       (Issue <a href="https://github.com/w3c/csswg-drafts/issues/6755">6755</a>)
     <li>Added ''::search-text'' as a new [=highlight pseudo-element=].