[css-pseudo-4] Clarify what 'normal cascade' means in note about ::selection defaults. w3c#1122

Author: fantasai

css-pseudo-4/Overview.bs

@@ -601,6 +601,7 @@ Cascading and Per-Element Highlight Styles</h3>
 
   Note: This paired-cascading behavior
   does not allow using the normal cascade
+  (i.g. '':root::selection'' rules in the <a>UA style sheet</a>)
   to represent the OS default selection colors.
   However it has been interoperably implemented in browsers
   and is thus probably a Web-compatibility requirement.