[css-pseudo-4] Pull highlight pseudo property logic from source comment into prose. #4401

Author: fantasai

css-pseudo-4/Overview.bs

@@ -507,9 +507,12 @@ Selecting Highlighted Content: the ''::selection'',  ''::inactive-selection'', '
 Styling Highlights</h3>
 
   The <a>highlight pseudo-elements</a> can only be styled
-  by a limited set of properties that do not affect layout.
-  <!-- ALSO: are not dependent on the exact bounds of the selection painting area,
-  and can be applied performantly in a highly dynamic environment-->
+  by a limited set of properties that do not affect layout
+  and can be applied performantly in a highly dynamic environment--
+  and additionally (to ensure interoperability)
+  whose rendering within the [[#highlight-bounds|required area]]
+  is not dependent on the exact (UA-determined) bounds
+  of the [=highlight overlay=].
   The following properties apply to the <a>highlight pseudo-elements</a>:
 
   <ul>