[css-pseudo-4] Take certain non-applicable but required properties fr…

…om the originating element #7591

css-pseudo-4/Overview.bs

@@ -894,16 +894,12 @@ Styling Highlights</h3>
     <wpt>
     textpath-selection-011.html
     </wpt>
-
-    <li>[=custom properties=] [[!CSS-VARIABLES-1]]
   </ul>
 
-  The 'forced-color-adjust' property cannot be set on [=highlight pseudo-elements=];
-  however a [=highlight pseudo-element=] must honor
-  any [=forced colors mode=] applied to its [=originating element=]
-  (and is therefore subject to the control of the [=originating element=]’s 'forced-color-adjust' value).
-
   Issue: Are there any other properties that should be included here?
+
+  Note: Historically (and at the time of writing)
+  only 'color' and 'background-color' have been interoperably supported.
 
   Note: The 'color' property sets the color of
   both the text
@@ -912,6 +908,25 @@ Styling Highlights</h3>
   applied to the text
   by the <a>originating element</a> and its ancestors and descendants.
 
+<h4>Values Resolved From Originating Elements</h4>
+  There are several properties that modify colors or resolve units but cannot be
+  used inside [=highlight pseudo-elements=], yet are important for specifyng the
+  appearance of properties that are used in the highlight. For example:
+
+  <ul>
+    <li>'forced-color-adjust' (used with [=forced colors mode=], which [=highlight pseudo-elements=] must respect)
+    <li>'font', 'font-size', etc. used to resolve 'em' and other font-relative units.
+    <li>'line-height' used to resolve 'lh' units.
+    <li>[=custom properties=] [[!CSS-VARIABLES-1]] used as property values.
+  </ul>
+
+  These properties are ignored if defined on the [=highlight pseudo-element=]
+  itself. Rather, any non-allowed property required to define the appearance of a
+  [=highlight pseudo-element=] is taken from the [=originating element=]’s
+  corresponding property. The computed style of the [=highlight pseudo-element=]
+  will include any properties taken from the originating element, even if not
+  allowed on the [=highlight pseudo-element=] itself.
+
 <!-- Add this back if for some reason someone wants to implement 'outline'?
   The outline, if supported, must be drawn
   around the union of the active portions of the <a>highlight overlay</a>
@@ -923,8 +938,6 @@ Styling Highlights</h3>
   into a single shape.
 -->
 
-  Note: Historically (and at the time of writing)
-  only 'color' and 'background-color' have been interoperably supported.
 
 <h3 id=highlight-ua-styles>
 Default UA Styles</h3>