If you are on a larger team you can be a language lawyer specialist in HTML and CSS, a handy person to have in a web dev shop.
Every person I know who's been the CSS specialist in a larger business has also had some design background or significant aptitude. Have you worked with someone who has just known CSS really well, with no programming language expertise, and without any design skills?
What do mean by language lawyer specialist? Actually, HTML, CSS and some JS is what i like doing. So you don't think, that these low level positions like HTML/CSS coder will be soon be replaced by a tool?
As another poster said, graphic design skills help too. For instance, coming up with good color schemes, maybe being able to draw, understanding how to use CSS properties to perfect character and line spacing to make it look right, etc.
But there is always a room for professional developers who are attention to details and are able to identity and to fix the problems.