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 point. Cheers
What are we to take from that? Do we try and hold that kind of selector in some do not use in case of emergency vault in our heads?
To try and get some real answers, I asked the smart folks who actually work on browsers what they think we should concern ourselves with in regards to CSS performance.
In the front-end world we are lucky that the Chrome Developer relations team are so accessible. However, I like balance. In addition, I reached out to people at Microsoft and Firefox and included some great input from WebKit too.