Add pointer to HTML about top layer and inertness
Once HTML defines how being on top of the top layer affects inertness, there should be a pointer from Fullscreen.
Currently, HTML hooks up dialog to the inert concept directly in https://html.spec.whatwg.org/multipage/interaction.html#blocked-by-a-modal-dialog, and not via top layer.
I think the proper fix here will be to move the language about inertness into the fullscreen standard, so that when something is at the top of the top layer, everything else is inert.
CSS WG discussed how fullscreen affects :modal in https://github.com/w3c/csswg-drafts/issues/7311#issuecomment-1163385752
RESOLVED: Fullscreen elements should inert the stuff behind them, and match :modal