A lot of great information here. I always knew that overqualifying selectors was bad, but it was always from a "clean code" point of view and not so much from a performance standpoint. Even more reason to avoid them now.

Although this is very interesting information, anyone reading this article should pay careful attention to the ending headline, "Is all this really necessary?" Odds are, unless you've produced some kind of CSS monstrosity, your CSS isn't the bottleneck in your website performance. Min and concat your Javascript, use appcache, local storage, etc. before prioritizing CSS performance.