It seems that it might be good to have a CSS pseudo-class to help mitigate the inherent FOUC with Web Components (now that they are asynchronous again).

The question is what should they be?

One idea is to have ":shadow-host", which simply indicates if the current element is a shadow host or not. This is specific to shadow DOM spec, which makes it applicable in non-Web Components world

Another idea would be to have ":component", which matches when an element is a custom DOM element.

Yet another thought is to just add ":components-ready", which only matches on document and shares the timing of some "componentsready" DOM event.