[selectors-4] Define functional pseudo-elements. #8268

Author: fantasai

selectors-4/Overview.bs

@@ -812,7 +812,9 @@ Syntax</h4>
 
 	The syntax of a <a>pseudo-element</a>
 	is "::" (two U+003A COLON characters)
-	followed by the name of the <a>pseudo-element</a> as an <a>identifier</a>.
+	followed by the name of the <a>pseudo-element</a> as an <a>identifier</a>,
+	and, in the case of a <dfn export id="functional-pseudo-element">functional pseudo-element</dfn>,
+	a pair of parentheses containing its arguments.
 	<a>Pseudo-element</a> names are <a>ASCII case-insensitive</a>.
 	No <a>white space</a> is allowed between the two colons, or between the colons and the name.