[css-pseudo-4] Tighten up default selection color requirement. w3c#4785

Author: fantasai

css-pseudo-4/Overview.bs

@@ -686,7 +686,7 @@ Cascading and Per-Element Highlight Styles</h3>
     </wpt>
 
 
-  The UA should use the OS-default highlight colors for ''::selection''
+  The UA must use its own highlight colors for ''::selection'' only
   when neither 'color' nor 'background-color' has been specified by the author.
 
   Note: This paired-cascading behavior