[fullscreen] `position` value of `::backdrop` pseudo-element (#27)

The spec says
> An element in a top layer has the following characteristics:
> ...
> * If its specified position property is static or relative, it computes to absolute.

But it doesn't mention whether that should happen for `::backdrop` pseudo-element.

I think for `::backdrop`, we should either also compute non-absolutely position values to `absolute`, or make the `position: fixed` rule be `!important`.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/27