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The suggested UA style sheet[1] has: dialog::backdrop { background: rgba(0,0,0,0.1); } So by default, the backdrop has height and width 0. I suspect most use cases of a modal dialog want a visible backdrop. Those authors will have to add something like: dialog::backdrop { position: fixed; top: 0; bottom: 0; left: 0; right: 0; } which is a bit cumbersome. How about adding such style to the suggested style sheet? [1] http://www.whatwg.org/specs/web-apps/current-work/multipage/rendering.html#flow-content-1
That's taken care of in: http://fullscreen.spec.whatwg.org/#user-agent-level-style-sheet-defaults
That sets styles for :fullscreen elements, which dialog generally isn't.
Ah, right. My bad.
Checked in as WHATWG revision r8197. Check-in comment: dialog::backdrop needs positioning, it isn't _that_ magical http://html5.org/tools/web-apps-tracker?from=8196&to=8197