Why are LLMs so bad with CSS?

I find they are much better at tailwind css. Been really impressed lately actually, and I was a skeptic

I find they're pretty good at some questions -- it can be so painful to look up authoritative answers in the almost 50 docs here:

https://www.w3.org/TR/css-2024/

I'd expect them to run into problems with complicated selectors though.

Can you give an example?