If it does support projecting differing CRSs though, please can you point me at it? I’m a nobody and nothing to do with qgis but I would love to have a play with it!
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!