Issue whatwg#625. Define Event.srcElement and Event.returnValue for c…

…ompat.

wpt: web-platform-tests/wpt#10258

dom.bs

@@ -431,6 +431,7 @@ would have been {{Event/AT_TARGET}}.
 interface Event {
   readonly attribute DOMString type;
   readonly attribute EventTarget? target;
+  readonly attribute EventTarget? srcElement; // historical
   readonly attribute EventTarget? currentTarget;
   sequence<EventTarget> composedPath();
 
@@ -446,6 +447,7 @@ interface Event {
 
   readonly attribute boolean bubbles;
   readonly attribute boolean cancelable;
+           attribute boolean returnValue;
   void preventDefault();
   readonly attribute boolean defaultPrevented;
   readonly attribute boolean composed;
@@ -580,6 +582,9 @@ string.
 <p>The <dfn attribute for=Event><code>target</code></dfn> attribute's getter must return the
 <a>context object</a>'s <a for=Event>target</a>.
 
+<p>The <dfn attribute for=Event><code>srcElement</code></dfn> attribute's getter must return the
+<a>context object</a>'s <a for=Event>target</a>.
+
 <p>The <dfn attribute for=Event><code>currentTarget</code></dfn> attribute must return the value it
 was initialized to. When an <a>event</a> is created the attribute must be initialized to null.
 
@@ -675,6 +680,12 @@ The <dfn attribute for=Event><code>bubbles</code></dfn> and
 <dfn attribute for=Event><code>cancelable</code></dfn> attributes
 must return the values they were initialized to.
 
+<p>The <dfn attribute for=Event><code>returnValue</code></dfn> attribute must be initialized to true
+when an <a>event</a> is created. When the attribute is set to false it must set the <a>canceled flag</a>
+if the {{Event/cancelable}} attribute value is true and the <a>in passive listener flag</a> is
+unset. The {{Event/returnValue}} attribute's getter must return false if the <a>canceled flag</a> is
+set, and true otherwise.
+
 The <dfn method for=Event><code>preventDefault()</code></dfn> method, when invoked, must set the
 <a>canceled flag</a> if the {{Event/cancelable}} attribute value is true and the
 <a>in passive listener flag</a> is unset.