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"Extensions to Document interface" add dictionary Options which has a HTMLTemplateElement field. Indirectly, register sets up an object that is required to implement HTMLElement (per the introduction, "The element prototype itself must inherit from the HTMLElement interface.") Wouldn’t it be neater to put these extensions on HTMLDocument?
There's no such thing as HTMLDocument in HTML spec: http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#the-document-object