... with the link taking the browser between three or four independent versions of the css file? Izzat how it works?
It's usually done with javascript. Google for "font size changer javascript" and you'll get lots of advice.

I don't bother doing that on sites I work on, because as Tim says, users can adjust things themselves using their browser's built-in abilities. But I have seen that little tool appearing on more sites. I just ignore it.

Rob