[css-pseudo-4] Define CSSPseudoElement.pseudo() w3c#3836

Author: fantasai

css-pseudo-4/Overview.bs

@@ -1194,6 +1194,7 @@ Additions to the CSS Object Model</h2>
         readonly attribute CSSOMString type;
         readonly attribute Element element;
         readonly attribute (Element or CSSPseudoElement) parent;
+        CSSPseudoElement? pseudo(CSSOMString type);
     };
   </pre>
 
@@ -1226,6 +1227,13 @@ Additions to the CSS Object Model</h2>
   {{CSSPseudoElement/parent}} will return a {{CSSPseudoElement}}
   while {{CSSPseudoElement/element}} returns an {{Element}}.
 
+  The <dfn attribute for=CSSPseudoElement>pseudo()</dfn> method
+  returns the {{CSSPseudoElement}} interface
+  representing the [=sub-pseudo-element=] referenced in its argument,
+  if such a [=sub-pseudo-element=] could exist and would be valid,
+  and null otherwise.
+  See {{Element/pseudo()}} below.
+
   Note: This interface may be extended in the future
   to other pseudo-element types
   and/or to allow setting style information