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"img, iframe, embed, object, video, audio, canvas, map. input, textarea, progress, meter"
Content fallback case for built-in elements is effectively an insertion point with a controlled "select" value. When the element chooses so, it may set it the value to "*" and allow fallback content to appear.
http://dvcs.w3.org/hg/webcomponents/rev/df110ec318b7