CSS Selectors Performance
“Aside from that, you are not allowed to care about the performance of * unless you concatenate all your javascript, have it at the bottom, minify your css and js, gzip all your assets, and losslessly compress all your images. If aren’t getting 90+ Page Speed scores, its way too early to be thinking about selector optimization.” – Amen!
Link: http://www.paulirish.com/2012/box-sizing-border-box-ftw/