The basic answer to whether to add a class to main is no. As you correctly noticed, according to the WHATWG specification, there can only be one tag per document (the currently visible one), so adding a class is redundant (even for BEM). Personally, when developing, I try to avoid even styling the main tag. But if I really need to style it, then I actually use main {}, since it's better than creating extra containers to avoid unnecessary DOM nesting.