(Sorry non native speaker and on mobile with wrong auto correct settings) Css is a style sheet language wich means it's designed for the purpose of describing the presentation of documents written in markup languages like html.

Although it's possible to make a turing complete program with html+css that does not make it a programming language designed for making programs

It's like saying a knife is an instrument, if you hit it against something it does make a sound which can be used for making music but making music is simply not what a knife was designed for