They are not allowed to loop or recurse, so no, by themselves, they are not (unless you accept repeating the function a potentially infinite amount of time).

CSS in general is Turing complete if you allow running multiple frames, although it requires ugly hacks. See e.g. https://codepen.io/propjockey/pen/dywNyBQ, which is quite insane IMO.

No. The implementation treats the function body as just another CSS rule set, which is applied to some virtual element. The ”result: somevalue;” rule then sets the ”result” style property on that element, and that property value gets plopped onto the call site.

Kinda clever, actually.

I remember there's expression() on IE6.

CSS Custom Functions are defined in a way they don't add anything over traditional CSS in this regard. I.e. they are just allowed to act as custom functions - not recursion mechanism, jump mechanisms, loop mechanisms, etc.

Stupider question: how long until it can run Doom?