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The UA style sheet for dialog includes [1]: dialog::backdrop { background: rgba(0,0,0,0.1); } But backdrop is not mentioned in the spec. [1] http://dev.w3.org/html5/spec/rendering.html#flow-content-1
It's defined in the Fullscreen spec: http://dvcs.w3.org/hg/fullscreen/raw-file/tip/Overview.html#::backdrop-pseudo-element
(In reply to comment #1) > It's defined in the Fullscreen spec: > http://dvcs.w3.org/hg/fullscreen/raw-file/tip/Overview.html#::backdrop-pseudo-element I think what comment 0 is saying is that every declaration of the default UA stylesheet should be in the rendering section, which makes sense.
Sorry if I wasn't clear in comment 0. I didn't realize backdrop was defined in the Fullscreen spec, so I'm OK with the WORKSFORME resolution.