[css-pseudo] Define ::backdrop

Right now ::backdrop is defined in https://fullscreen.spec.whatwg.org/#::backdrop-pseudo-element

Each element in a top layer has a ::backdrop pseudo-element. This pseudo-element is a box rendered immediately below the element (and above the element before the element in the set, if any), within the same top layer.
It does not inherit from any element and is not inherited from. No restrictions are made on what properties apply to this pseudo-element either.

Well, there are several details that are not clear to me:

• Is it a tree-abiding pseudo-element?
• If it's a tree-abiding pseudo-element, where is it originated? Before ::marker, between ::marker and ::before, between ::before and 1st child, between last child and ::after, after ::after?
• It accepts random properties, e.g. display: list-item. But then:
  • Does it really become a list item?
  • If it becomes a list item, does it increment the implicit list-item counter as usual? If so, it's position in the tree matters.
  • Do counter properties even work in ::backdrop? The properties are defined as applying to "all elements (includes ::before and ::after pseudo-elements)", but the ::backdrop definition says "no restrictions".
  • Does it originate a ::marker pseudo-element?
  • Should ::backdrop::marker be a valid selector?

I think css-pseudo should define ::backdrop and clarify these details.

Start with the css-pseudo specification and the Fullscreen specification's ::backdrop definition cited in the issue. Resolve the listed questions about tree position, applicable properties, counters, list items, and ::marker, then update the specification with normative definitions covering those behaviors.