Ah gotcha, thanks - I know next to nothing about css geometry itself, but was curious to play with it, and think I had an imagine in my mind that was different to what you've created.

For what it's worth, I just skimmed the MDN docs and e.g. DOMPoint is using the right kinda wording to sound like it's not inherently cartesian-only. Could be interesting, that. Thanks for getting it into my head!