The definition of the video-width media feature (same thing for video-height) includes this:

lengths are interpreted according to [[#units]].

This means (I believe) that we're doing things like measuring in CSS px, not in device pixels, as we always do everywhere.

However, I suspect that this isn't actually what implementations plan to do, and a 4K TV probably wants to report 3840 × 2160 (and a resolution of 1dppx), regardless of viewing distance.

If so, we should probably change the above phrasing, or at the very least add a note saying that implementations are likely to anchor the css px unit to physical device pixels rather than to the angular definition.

If we do intend the usual unit gymnastics, the above phrasing is fine, but a note should probably be added, as this may be unexpected / surprising.