As a baby step towards defining hit testing that would be useful in other places as well, a css spec should define some anchoring terminology for the area encompassed by the border of an element, taking corner shaping (by border-radius and possible future properties) into account.

Quite possibly the term border edge is the one I'm after, but it is defined in CSS2.1 without mentioning border-radius, then effectively monkey-patched in CSS Backgrounds 3 §5.1 to say that it is affected by border radius. That spec should probably provide an updating definition of border edge that states explicitly states it takes corner shaping into account(corner shaping should probably be an anchor term as well), and then let border-radius state how does the corner shaping.

While we're at it, we should also clarify whether the border box takes corner shaping into account.

This would have been useful at least in in https://drafts.csswg.org/css-ui-3/#cursor, and in https://wicg.github.io/spatial-navigation/#heuristics