[selectors-4] Sigh, document the ::-webkit-* parsing quirk. #3051

Author: tabatkins

selectors-4/Overview.bs

 	
 
 
+
+Appendix B: Obsolete but Required Parsing Quirks for Web Compat
+
+	This appendix is normative.
+
+	Due to legacy Web-compat constraints,
+	User Agents expecting to parse Web documents must support the following features:
+
+	* All [=pseudo-classes=] and [=pseudo-elements=] whose name begins with the string “-webkit-”
+		(matched ASCII case-insensitively)
+		must be treated as valid at parse time.
+		If they're not otherwise recognized and supported,
+		they must be treated as matching nothing.
+
+
 
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