::backdrop and :fullscreen now defined in CSS specs

[tidoust]

The ::backdrop pseudo-element is now defined in CSS Positioned Layout 4 with a note to mention that Fullscreen extends the definition. Similarly, the :fullscreen pseudo-class is defined in Selectors 4.

Ideally, there would be only and only one definition flagged as "exported" across specs. Could the definitions in Fullscreen switch to "internal" with links added to the definitions in the CSS spec?

[annevk]

That is the plan, yes: whatwg/html#9093.