If it was a markup language, it would be much easier to use. Instead, it's a rules based system that works upon a markup language (and I'm not trying to be pedantic, it would be easier if it was part of the markup, but also require much more work to change the look of content by needing to change the markup - which might be generated dynamically and difficult to change without lots of programming effort).
https://developer.mozilla.org/en-US/docs/Learn/CSS/First_ste...