[css-pseudo-4] Forbid UA tweaks to author colors #6853

Author: fantasai

css-pseudo-4/Overview.bs

@@ -952,6 +952,17 @@ Styling Highlights</h3>
   into a single shape.
 -->
 
+  If, for a given [=highlight pseudo-element=],
+  there are colors specified in the [=author origin=],
+  those colors must be respected as specified;
+  i.e. the UA must not apply any extra processing
+  (such as using semi-transparent washes).
+  However if there are no colors in the [=author origin=],
+  the UA may apply additional color processing.
+
+  Note: This is to ensure that color contrast is preserved
+  across all user agents interpreting a given author stylesheet.
+
   Vendor-prefixed properties such as '-webkit-text-fill-color'
   are not applicable to the [=highlight pseudo-elements=].
   <!-- https://github.com/w3c/csswg-drafts/issues/7580 -->