You cannot write a program in CSS. It is not a programming language.
Look:
Computer science precisely classifies things that are almost computers.
There's a hierarchy that has nothing to do with clout. It's a genuine field of study. It's even one of the freaky ones where the landmark name isn't a computer scientist, but Noam yes-that-one Chomsky. The linguist.
We say a type of machine recognizes a language. Famously, "you can't parse HTML with regexes," because regular expressions are a specific grammar that does not allow self-reference. Added regex features like lookahead only extend it into context-sensitive grammar.
I'm pretty sure HTML is even less than that. This comment sent me sixteen Wikipedia tabs deep to double-check, and I started drinking around number eight. But I feel confident saying you also could not parse a regex, with HTML. Even the delightful wackaloons who get Powerpower to act Turing-complete could only half-ass it out of HTML by including CSS and a human hitting tab-space-tab-space.
It's not a value judgement, when we say CSS and HTML are not programming languages. And we're not just being pedantic toward you. Again: have you seen what we say about Javascript? This is how we are.