[css-pseudo-4] Define CSSPseudoElement.parent (vs CSSPseudoElement.element) for sub-pseudo-elements w3c#3836

Author: fantasai

css-pseudo-4/Overview.bs

@@ -1193,6 +1193,7 @@ Additions to the CSS Object Model</h2>
     interface CSSPseudoElement : EventTarget {
         readonly attribute CSSOMString type;
         readonly attribute Element element;
+        readonly attribute (Element or CSSPseudoElement) parent;
     };
   </pre>
 
@@ -1214,7 +1215,16 @@ Additions to the CSS Object Model</h2>
   </dl>
 
   The <dfn attribute for=CSSPseudoElement>element</dfn> attribute is the
+  [=ultimate originating element=] of the [=pseudo-element=].
+
+  The <dfn attribute for=CSSPseudoElement>parent</dfn> attribute is the
   [=originating element=] of the pseudo-element.
+  For most pseudo-elements
+  {{CSSPseudoElement/parent}} and {{CSSPseudoElement/element}}
+  will return the same {{Element}};
+  for [=sub-pseudo-elements=],
+  {{CSSPseudoElement/parent}} will return a {{CSSPseudoElement}}
+  while {{CSSPseudoElement/element}} returns an {{Element}}.
 
   Note: This interface may be extended in the future
   to other pseudo-element types