Should we worry about CSS selectors?

The question was essentially, Should authors concern themselves with the selectors used in relation to CSS performance?

Let's start at the beginning, where things like the CSSOM and DOM actually get constructed. Paul Lewis (http://aerotwist.com/), Developer Advocate for Chrome Developer Relations explains, Style calculations are affected by two things: selector matching and the size of the invalidation. When you first load a page all the styles need to be calculated for all the elements, and that's a function of tree size and the number of selectors.

For more detail, Lewis quotes Rune Lillesveen (https://docs.google.com/document/d/1vEW86DaeVs4uQzNFI5R-_xS9TcS1Cs_EUsHRSgCHGu8/edit#) on the Opera team (who does a lot of work on Blink's style code):

At the time of writing, roughly 50% of the time used to calculate the computed style for an element is used to match selectors, and the other half of the time is used for constructing...