For certain layouts and CSS features to work you need to add structure for it AKA additional tags, typically divs and spans because they have no meaning in term

For certain layouts and CSS features to work you need to add structure for it AKA additional tags, typically divs and spans because they have no meaning in terms of your document. So your document becomes a mix of semantic structure and then wrappers, containers, pushers, purely visual areas and so on. Also you're going to have things that are specifically redundant for your use-case, but you still add them for consistency and re-usability. CSS would have to be quite a bit more powerful to achieve a clean separation of document structure and layout. It would need things like purely visual grouping mechanisms, reactive or backtracking selectors and HTML itself would probably need to be either a relational or graph structure instead of a tree to actually convey the semantics correctly and consistently.