explicitly spec behaviour of registered customs with inherits = false

Author: delan

css-pseudo-4/Overview.bs

@@ -796,8 +796,9 @@ Cascading and Per-Element Highlight Styles</h3>
   When any supported property is not given a value by the cascade,
   its value is determined by inheritance from
   the corresponding <a>highlight pseudo-element</a>
-  of its <a>originating element</a>'s parent element
-  (regardless of whether that property is an <a>inherited property</a>).
+  of its <a>originating element</a>'s parent element.
+  This occurs regardless of whether that property is an <a>inherited property</a>,
+  including [=registered custom properties=] where the [=inherits descriptor=] is false.
 
     <wpt>
     css/css-pseudo/active-selection-051.html