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In http://parkour.mtv/webcomponents/spec/custom/index.html#dfn-create-custom-element-declaration, "If BASE is empty, let INTERFACE be HTMLElement Otherwise, let INTERFACE be the element interface for BASE and HTML Namespace" Suppose that the BASE is also a custom element, and its declaration hasn't been loaded yet. This will turn this declaration into a dud. That's bad.
Actually working link: https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/custom/index.html#dfn-create-custom-element-declaration