I don't know if CSS is underrated, but it's definitely a speciality that a lot of software developers don't (want to) specialize in. It's like the front-end equivalent of SQL; so much is possible in SQL, but most logic is built in application code because SQL is scary.

Most logic is built in application code because it makes it easy to structure, reason about, and test. Developers should be scared of code that is hard to test and reason about.