At an initial look I agree with you that the functions look very awkward. But I thought the same about CSS variables until I started using them and now I think they’re great so I’ll keep an open mind. I almost wonder if they would have been better off calling them “templates”, or something. “Functions” is going to make people think it’s just like JS.
Perhaps I should also be sympathetic to the spec authors, since they're working under tight constraints of language design. Things might look a lot more elegant if HTML/CSS/JS were designed from scratch as a coherent whole.
https://developer.mozilla.org/en-US/docs/Web/API/CSS_Paintin...
It’s Chrome only though.
I've always wished more of the underlying CSS functionality were exposed to the scripting side, like calculating flex/grid layouts, without necessarily tying it to performance-critical painting path. Usually more expressive power means more responsibility, but there are already web APIs that do it in a controlled way, like request animation frame. So I can appreciate the potential of the Painting API and others in a similar direction.
At the same time, I also wish there was effort in the complete opposite way, to start over "from scratch" and make it easier (or possible at all) for people to build alternative browsers with a minimal subset of CSS. Perhaps something like a modularization of web technology, where the current hugely complex system is broken down into reusable libraries for building small and large browser-like applications. I know something like that exists, like Servo etc., but much smaller and simpler building blocks.