[fullscreen] ::backdrop of root (#24)

>From the current spec, if we make the root element fullscreen, it would have a black background, because root has a transparent background by default, and the `::backdrop` will then provide the background color.

It doesn't match any current implementation, and it doesn't really make sense to me.

I propose that we change `*|*:fullscreen::backdrop` in the default style sheet to `*|*:not(:root):fullscreen::backdrop`.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/24