[css-pseudo-4] Explicitly state that UAs can do extra effects to highlighted content. w3c#5522

Author: fantasai

css-pseudo-4/Overview.bs

@@ -569,6 +569,12 @@ Default UA Styles</h3>
     :root::target-text    { color: MarkText; background: Mark; }
   </code></pre>
 
+  UAs may apply additional effects to enhance the presentation
+  of highlighted content,
+  for example dimming content other than the highlighted text
+  or transitioning out a highlight style based on user interactions or timing.
+  These are not controlled by CSS.
+
 <h3 id=highlight-bounds>
 Area of a Highlight</h3>