[css-pseudo] no decoration propagation on highlights (closes #6829)

Author: delan

css-pseudo-4/Overview.bs

@@ -989,6 +989,23 @@ Text and Text Decorations</h4>
     </div>
   </div>
 
+  While the element’s own
+  <a href="https://www.w3.org/TR/css-text-decor/#line-decoration">line decorations</a>
+  are propagated from
+  containing <a spec=css-text-decor>decorating boxes</a>
+  as usual,
+  decorations introduced by [=highlight pseudo-elements=]
+  apply only to the text
+  directly associated with the [=originating element=]
+  (or intervening [=pseudo-element=] such as ''::first-line'' or ''::first-letter'').
+
+  Note: Because highlight decorations
+  are propagated through inheritance,
+  not through the box tree,
+  they can be declared on the root element
+  and still reliably decorate out-of-flow descendants,
+  which is not the case for non-highlight decorations.
+
 <h4 id="highlight-replaced">
 Replaced Elements</h4>