Summary

If you've developed on the Web for any non-trivial period of time you will know that the answer to most web related questions is it depends. I hate that there are no simple, cast-iron rules in relation to CSS performance that can be banked upon in every situation. I'd genuinely love to write those rules out here in a nice little paragraph and believe they would be universally true. But I can't because there simply aren't any universal cast-iron truths in relation to performance. There can't ever be any because there are simply too many variables. Engines update, layout methods become optimised, every DOM tree is different, all CSS files are different. On and on ad infinitum. You get the picture.

I'm afraid the best I can offer is to not sweat things like CSS selectors or layout methods in advance. It's unlikely they will be your problem (but, you know, they just might). Instead, concentrate on making the thing. Then, when the thing is...