Please also add a :buffering (or :waiting) pseudo-class distinct from :stalled, at the same time as adding :stalled.
Use-case: showing loading/buffering spinner, often with a %, e.g. on Netflix.
I’d like to see :buffering defined similar to :stalled https://html.spec.whatwg.org/multipage/media.html#event-media-stalled, however with the key difference that data *is* forthcoming. Similar to how the waiting event is defined: https://html.spec.whatwg.org/multipage/media.html#event-media-waiting
Alternatively I'd be ok with a :waiting pseudo-class that maps to the waiting event/definition (might be simpler for the platform / web-developers if we "just" do/keep a direct mapping of the pseudo-class to an existing HTML event name).
I do think it is important to add :buffering (or :waiting) at the same time as adding :stalled, in order to avoid having :stalled be errantly overloaded by implementers and/or web developers to handle the buffering use-case.