[css-pseudo] clarify highlight color:currentColor (closes w3c#6818)

Author: delan

css-pseudo-4/Overview.bs

@@ -935,7 +935,7 @@ Text and Text Decorations</h4>
 
   For this purpose,
   ''currentColor'' on a [=highlight pseudo-element=]’s 'color' property represents
-  the 'color' of the next <a>highlight pseudo-element</a> layer below,
+  the 'color' of the next <em>active</em> <a>highlight pseudo-element</a> layer below,
   falling back finally to the colors that would otherwise have been used
   (those applied by the [=originating element=] or
   an intervening [=pseudo-element=] such as ''::first-line'' or ''::first-letter'').
@@ -1295,6 +1295,18 @@ Additions to the CSS Object Model</h2>
   See <a href="https://github.com/w3c/csswg-drafts/issues/3607">Issue 3607</a>
   and <a href="https://github.com/w3c/csswg-drafts/issues/3603">Issue 3603</a>.
 
+<h3 id="getComputedStyle">
+{{Window/getComputedStyle()}}</h3>
+
+When the second parameter <var>pseudoElt</var>
+refers to a [=highlight pseudo-element=],
+{{Window/getComputedStyle()}} returns styles
+as if that highlight is active
+and all other highlights are inactive.
+This avoids the potential
+ambiguity and privacy risks of returning a result
+that depends on the actual highlight state.
+
 <h2 id="css2-compat">
 Compatibility Syntax</h2>