why do you think the js interpreter is that special compared to all the rest? AFAIU, CSS is a much more complex beast, as the spec has not been written to reflect the way it could be implemented

Writing JS JITs is a specialized skill that requires a deep knowledge of compilers, JavaScript, and CPU architectures.

CSS is complicated but it's not as complex. It's just a matter of throwing enough sufficiently competent developers at the problem.

Ladybird doesn't even have a JIT right now. They used to, but it got taken out because as best as I can tell, nobody on the project knows how to write one.