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http://www.whatwg.org/specs/web-apps/current-work/multipage/interactive-elements.html#attr-menuitem-command The spec doesn't state the required microsyntax of the <menuitem> "command" attribute -- what values for it are valid. From the spec text and examples, I can infer that it seems like the value of the "command" attribute must a single ID of another element in the document. But the spec never explicitly states that anywhere.
It actually does, but I agree that it wasn't obvious. I've added a note to make it clearer (and also fixed the definition a bit).
Checked in as WHATWG revision r8026. Check-in comment: Make a note that this is where command='''s syntax is defined, and also make <menuitem> something that can be legally given in a detached subtree http://html5.org/tools/web-apps-tracker?from=8025&to=8026