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The prose about event processing for beforeunload says 'If the return value is null, then cancel the event.' However, the WebIDL definition for OnBeforeUnloadEventHandlerNonNull is |callback OnBeforeUnloadEventHandlerNonNull = DOMString (Event event);| which has a non-nullable DOMString return value. It should be |callback OnBeforeUnloadEventHandlerNonNull = DOMString? (Event event);|.
Checked in as WHATWG revision r8282. Check-in comment: OnBeforeUnloadEventHandler is supposed to allow null return values (and some markup error fixes, oops) http://html5.org/tools/web-apps-tracker?from=8281&to=8282