I think this could definitely help with some of the bulk testing which would be fantastic. It seems like it might be difficult to test really complex CSS (which is the stuff that probably needs the most testing) like drop-down menus, interactive hover effects, etc.

I've definitely had those times where we've tweaked a stylesheet only to find out later that some page deep in the site now caused a floated div to move to the next line and mess up the layout. It would be so great to have unit testing to keep an eye on these kinds of things.