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As communications section has been split out from HTML5 spec, we no longer have reference for definition of MessageEvent. How about referring HTML5 Web Messaging spec? http://dev.w3.org/html5/postmsg/
You're better off using the full spec: http://whatwg.org/html I've added a reference to the spec that defines it, anyway.
Checked in as WHATWG revision r6747. Check-in comment: refs http://html5.org/tools/web-apps-tracker?from=6746&to=6747