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Interface objects are Functions with a [[Call]] behavior that throws by default. http://dev.w3.org/2006/webapi/WebIDL/#es-interface-call Therefore typeof HTMLElement === "function".
That should already be required, since http://dev.w3.org/2006/webapi/WebIDL/#interface-object says that the interface object is a function object, and http://dev.w3.org/2006/webapi/WebIDL/#dfn-function-object describes those. (It has some out of date ES6 terminology that needs updating, but hopefully it is still clear.)
Do you just want a clarifying note pointing out that "typeof <interfaceobject>" will be function? I can add that, though I think it would be better place in the preceding section http://dev.w3.org/2006/webapi/WebIDL/#interface-object.
I just thought a note would be useful since more than one smart person failed to come to this conclusion after reading the spec.
I've added a note: http://dev.w3.org/cvsweb/2006/webapi/WebIDL/Overview.xml.diff?r1=1.680;r2=1.681;f=h http://dev.w3.org/cvsweb/2006/webapi/WebIDL/v1.xml.diff?r1=1.122;r2=1.123;f=h