[selectors-4] Functional ::-webkit- pseudos aren't auto-valid. Also specify serialization, since annevk pointed out it's unclear.

Author: tabatkins

selectors-4/Overview.bs

@@ -3798,10 +3798,16 @@ Appendix B: Obsolete but Required Parsing Quirks for Web Compat</h2>
 
 	* All [=pseudo-elements=] whose names begin with the string “-webkit-”
 		(matched <a>ASCII case-insensitively</a>)
+		and who are not functional notations
 		must be treated as valid at parse time.
+		(That is, <css>::-webkit-asdf</css> is valid at parse time,
+		but <css>::-webkit-jkl()</css> is not.)
 		If they're not otherwise recognized and supported,
 		they must be treated as matching nothing.
 
+		These otherwise-unsupported pseudo-elements
+		must be serialized in ASCII lowercase.
+
 
 <h2 id="changes">
 Changes</h2>