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It's sort of implied by the document.register spec: "If PROTOTYPE is null, let PROTOTYPE be the interface prototype object for the HTMLSpanElement interface", but not said explicitly. Actually, I don't see "extends" mentioning its relation to the prototype, and the relationship bewtween HTMLElementElement and document.register does not seem to be clearly specified (as far as I can tell -- apologies if I missed it).
This is now fixed.