[css-pseudo] highlight pseudos and custom properties

Author: delan

css-pseudo-4/Overview.bs

@@ -50,6 +50,11 @@ spec:fill-stroke-3; type:property; text:stroke-width
   and on their syntax and interaction with other <a>selectors</a>,
   see [[!SELECTORS-4]].
 
+<h2 id="styling">Styling Pseudo-elements</h2>
+
+  [=Custom properties=] [[!CSS-VARIABLES-1]] are always allowed in pseudo-element styles,
+  even if the pseudo-element has a restricted set of CSS properties applying to it.
+
 <h2 id="typographic-pseudos">
 Typographic Pseudo-elements</h2>