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"Function register" <-- See http://lists.w3.org/Archives/Public/public-webapps/2012OctDec/thread.html#msg665
AFAIK "Function" is not a thing in Web IDL. FWIW Blink has moved to this: callback function CustomElementConstructor = Element(); as can be seen here <http://src.chromium.org/viewvc/blink/trunk/Source/core/dom/Document.idl?r1=152430&r2=152429&pathrev=152430>