The only time I've ever seen CSS make a difference to page rendering speed is the use of large drop shadows (e.g. 25px wide, on page-size elements) on large DOM elements. Just five of them kills scrolling performance.
Anything else, page rendering times for me have only ever been slowed by too many DOM elements (> 1000). I've never come across a real-world case of the actual selector syntax making a difference.
Anything else, page rendering times for me have only ever been slowed by too many DOM elements (> 1000). I've never come across a real-world case of the actual selector syntax making a difference.