[css-conditional-3] Define @supports selector function.

Depends on #3773.

Closes #3207.

css-conditional-3/Overview.bs

@@ -245,8 +245,9 @@ with <<supports-condition>> defined as:
 	                     | <<supports-in-parens>> [ and <<supports-in-parens>> ]*
 	                     | <<supports-in-parens>> [ or <<supports-in-parens>> ]*
 	<dfn>&lt;supports-in-parens></dfn> = ( <<supports-condition>> ) | <<supports-feature>> | <<general-enclosed>>
-	<dfn>&lt;supports-feature></dfn> = <<supports-decl>>
+	<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>> | <<supports-decl>>
 	<dfn>&lt;supports-decl></dfn> = ( <<declaration>> )
+	<dfn>&lt;supports-selector-fn></dfn> = selector( <<complex-selector>> )
 </pre>
 
 The above grammar is purposely very loose for forwards-compatibility reasons,
@@ -286,6 +287,12 @@ as follows:
 ::
 	The result is true if the UA [=supports=] the declaration within the parentheses.
 
+: <<supports-selector-fn>>
+::
+	The result is true if the UA
+	<a href="#dfn-support-selector">supports the selector</a>
+	provided as an argument to the function.
+
 : <<general-enclosed>>
 ::
 	The result is unknown.
@@ -472,6 +479,12 @@ compound values; implementations must implement all parts of the value
 in order to consider the declaration supported, either inside a style rule
 or in the declaration condition of an ''@supports'' rule.
 
+A CSS processor is considered to
+<dfn export for=CSS id="dfn-support-selector">support a CSS selector</dfn>
+if it accepts that selector (rather than discarding it as a
+parse error), and that selector doesn't contain
+<a>unknown -webkit- pseudo-elements</a>.
+
 <h2 id="apis">APIs</h2>
 
 <h3 id='extentions-to-cssrule-interface'>