[selectors-4] Sigh, document the ::-webkit-* parsing quirk. w3c#3051

Author: tabatkins

selectors-4/Overview.bs

@@ -3788,6 +3788,21 @@ Appendix A: Guidance on Mapping Source Documents &amp; Data to an Element Tree</
 	</div>
 
 
+<h2 id="compat">
+Appendix B: Obsolete but Required Parsing Quirks for Web Compat</h2>
+
+	<em>This appendix is normative.</em>
+
+	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 <a>ASCII case-insensitively</a>)
+		must be treated as valid at parse time.
+		If they're not otherwise recognized and supported,
+		they must be treated as matching nothing.
+
+
 <h2 id="changes">
 Changes</h2>