He does know how specificity works, I guess. But he is talking about reusability and inheritance. Still I find the claim way too harsh. I use id's in my CSS occ

He does know how specificity works, I guess. But he is talking about reusability and inheritance. Still I find the claim way too harsh. I use id's in my CSS occassionally. For #wrapper or #carroussel maybe. It it premature optimization to (re-)write everything to classes. Your HTML should be leading.