[fullscreen] Fullscreen element with `display: contents` (#28)

The CSS Display Module Level 3 spec adds a new value to `display` property which is [`contents`](https://drafts.csswg.org/css-display-3/#box-generation). It is currently only supported by Firefox.

This value, like `none`, doesn't generate any layout box for the element, and is not affected by the `position` value. So if requesting fullscreen on element with this setting, it is not possible to render this element in the top layer.

In this case, I suggest that the element should be rendered normally as if it is not in the top layer, and the `::backdrop` pseudo-element should not be rendered.

Not sure what the spec should say.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/28