A few quick searches discovered that I'm obviously not the first person to have the thought "Can I unit test my CSS?".
I'm wondering if anyone here has taken on CSS Unit Testing successfully? If you've tried and failed, or just have your own theories, please tell me why it (apparently) hasn't been done yet?
It doesn't make sense to do so IMO. CSS is a language for controlling page layout; trying to unit-test it would be like trying to unit-test Postscript.
The only obvious use for unit testing I could think of is comparing screenshots to see which browsers were affected by a change in the code. Beyond that though, you can't say the code is right unless you manually compare everything in the rendered output to what the spec says it should be. To me, getting CSS right is always going to be trial-and-error.
You could use Selenium, which is a web testing framework. This way you could assert what styles are to be applied to elements in the DOM etc.
Otherwise, not sure what your aim is. Unit testing is, as the name suggests, testing 'Units', and to me, this only makes sense with code, not css.
You can unit test the application of your CSS (i.e., are the styles/classes applied properly). You can visually inspect the results of applying your CSS. I don't see how you can unit test the CSS itself, since the CSS does not "execute." As @claimid (do you have a real name?) suggests, Selenium is one tool you can use to test your UI, though I wouldn't call that unit testing either. Note that not all tests are "unit" tests nor do they need to be.
Currently, understanding whether CSS is working properly or not is visually determined. So far, the best way to test it is through installing multiple browsers on your dev machine. Start using tools like Firebug and Web Developer on Firefox, and forget about Unit testing CSS until it becomes Turing complete. :-)
I'm trying to make unit test for CSS something real. http://github.com/gagarine/CSSunit I will implement that as a jQuery Plugin but I'm sure it will be easy to port.