[css-pseudo-4] Add CSSPseudoElement.element attribute

Fixes #2816.

Author: birtles

css-pseudo-4/Overview.bs

@@ -838,13 +838,14 @@ Interface CSSPseudoElement</h3>
     [Exposed=Window]
     interface CSSPseudoElement {
         readonly attribute CSSOMString <a href="#dom-csspseudochild-type">type</a>;
+        readonly attribute Element element;
         readonly attribute <a href="https://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleDeclaration">CSSStyleDeclaration</a> <a href="#dom-csspseudochild-style">style</a>;
     };
 
     CSSPseudoElement implements EventTarget;
   </pre>
 
-  The <dfn id="dom-csspseudochild-type">type</dfn> attribute
+  The <dfn attribute for=CSSPseudoElement>type</dfn> attribute
   is a string representing the type of the pseudo-element.
   This can be one of the following values:
 
@@ -863,7 +864,10 @@ Interface CSSPseudoElement</h3>
 
   Issue: Why are these strings different from the name of the pseudo-element?
 
-  The <dfn id="dom-csspseudochild-style">style</dfn> attribute
+  The <dfn attribute for=CSSPseudoElement>element</dfn> attribute is the
+  [=originating element=] of the pseudo-element.
+
+  The <dfn attribute for=CSSPseudoElement>style</dfn> attribute
   is a <code>CSSStyleDeclaration</code> [[!CSSOM]]
   that allows directly setting style information (inline styles) onto the pseudo-element.
   Inline styles on a <code>CSSPseudoElement</code> have precedence over all
@@ -949,6 +953,8 @@ Changes</h2>
     <li>Links to pseudo-element OM design discussions.
     <li>Various minor clarifications.
     <li>Use ''spelling-error'' and ''grammar-error'' with ''::spelling-error'' and ''::grammar-error'' in the UA stylesheet.
+    <li>Added the {{CSSPseudoElement/element}} attribute to the
+    {{CSSPseudoElement}} interface.
   </ul>
 
 <h2 class="no-num" id="acknowledgements">