[css-pseudo-4] Explain something about what kind of properties are allowed, for future reference.

Author: fantasai

css-pseudo-4/Overview.bs

@@ -473,6 +473,10 @@ Selecting Highlighted Content: the ''::selection'',  ''::inactive-selection'', '
 <h3 id="highlight-styling">
 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-->
   The following properties apply to the <a>highlight pseudo-elements</a>:
 
   <ul>