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http://www.whatwg.org/specs/web-apps/current-work/multipage/interactive-elements.html#attr-menuitem-radiogroup The spec doesn't state the required microsyntax of the <menuitem> "radiogroup" attribute -- what values for it are valid. From the spec text and examples, I can infer that it seems like the value of the "radiogroup" attribute is text. But the spec never explicitly states that anywhere.
"Except where otherwise specified, attributes on HTML elements may have any string value, including the empty string. Except where explicitly stated, there is no restriction on what text can be specified in such attributes." -- http://whatwg.org/html#attributes But I've restricted radiogroup="" to not allow the empty string, which will make this more obvious as a side-effect.
Checked in as WHATWG revision r8027. Check-in comment: Restrict <menuitem radiogroup> to not allow the empty string (since it'll look like it's different than the absence of the attribute, but those two cases are treated the same) http://html5.org/tools/web-apps-tracker?from=8026&to=8027