[selectors] Export the definition of unknown "-webkit-" pseudo-elements.

I want to reference it from css-conditional, in order to define the @supports
selector function.

Author: emilio

selectors-4/Overview.bs

@@ -3841,9 +3841,10 @@ Appendix B: Obsolete but Required Parsing Quirks for Web Compat</h2>
 		(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.
+		they must be treated as matching nothing, and are
+		<dfn export>unknown -webkit- pseudo-elements</dfn>.
 
-		These otherwise-unsupported pseudo-elements
+		<a>Unknown -webkit- pseudo-elements</a>
 		must be serialized in ASCII lowercase.
 
 		<details class=note>