[selectors-4] Define legacy selector aliases. w3c#5847

Author: fantasai

selectors-4/Overview.bs

@@ -1031,6 +1031,13 @@ Invalid Selectors and Error Handling</h3>
 
 	An <a>invalid selector</a> represents, and therefore matches, nothing.
 
+<h3 id="legacy-aliasing">
+	Legacy Aliases</h3>
+
+	Some selectors have a <dfn>legacy selector alias</dfn>.
+	This is a name which, at parse time, is converted to the standard name
+	(and thus does not appear anywhere in any object model representing the selector).
+
 <h2 id="logical-combination">
 Logical Combinations</h2>
 
@@ -1135,7 +1142,7 @@ The Matches-Any Pseudo-class: '':is()''</h3>
 	As previous drafts of this specification
 	used the name <dfn>:matches()</dfn> for this pseudo-class,
 	UAs may additionally implement this obsolete name
-	as an alias for '':is()''
+	as a [=legacy selector alias=] for '':is()''
 	if needed for backwards-compatibility.
 
 <h3 id="negation">