The CSSWG recently resolved to add a :modal pseudo class that applies to elements that are "modal". That was general purpose, in that it should apply to anything that fits the description of being "modal", not just modal <dialog>.

Today, the OpenUI WC similarly resolved to add a :top-layer pseudo class that should apply to (at least) elements using the Popup API which are currently in the top layer. The intention for the naming and behavior, though, was that this pseudo class should also be general purpose. It should match any type of element in the top layer, including modal <dialog>, fullscreen elements, and ::backdrop pseudo elements. Also maybe Shared Element Transitions?

Thoughts?