Just to pile on what you said, I believe that with Flex and other CSS rules, the DOM tree is less deep then it used to be (less wrappers for alignments). Obvio

Just to pile on what you said, I believe that with Flex and other CSS rules, the DOM tree is less deep then it used to be (less wrappers for alignments). Obviously, I don't work at the scale of Facebook, but it's a tendency that I have witnessed in different projects. So it's possible that the speed tradeoff between classes and ids become irrelevant, if it's not already.