CSS is not a "real" programming language in that it is not Turing complete but boy is there a lot to know about it. If you believe, as I do, that the best way to master a complex technology (when there is good documentation) is to master the official documentation, CSS makes you work pretty hard as

https://www.w3.org/TR/css-2023/

references roughly 50 documents with a bit of the mind-numbing structure that "Document Y overrides Document X".