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"When an unresolved element is created, its element interface must be: the HTMLElement, if the namespace is HTML Namespace; the SVGElement, if namespace is SVG Namespace; or" This is not quite right. I should take type extensions into the account. For <button is="foo-bar">, the element interface must be HTMLButtonElement.
https://dvcs.w3.org/hg/webcomponents/rev/bd93a00feb0a