Browser representatives on CSS performance

While I generally never worry CSS selectors when authoring a style sheet (typically I just put a class on anything I want to style and select it directly) every so often I see comments from people way smarter than me that relate specifically to a certain selector. Here's a quote from Paul Irish (https://www.paulirish.com/) in relation to a post on A List Apart from Heydon Pickering (http://alistapart.com/article/quantity-queries-for-css) which used a specific type of selector:

These selectors are among the slowest possible. ~500 slower than something wild like div.box:not(:empty):last-of-type .title”. Test page http://jsbin.com/gozula/1/quiet That said, selector speed is rarely a concern, but if this selector ends up in a dynamic webapp where DOM changes are very common, it could have a large effect. So, good for many use cases but keep in mind it might become a perf bottleneck as the app matures. Something to profile at that...