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

"It, and its ::backdrop pseudo-element, are not rendered if its inclusive ancestor has the display property set to none." is a duplicate if `::backdrop` already has the same requirements. I think we should fix this as suggested by @upsuper above, instead of having the `::backdrop` section act as if it were an element for the purposes of this section.

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