CSS only needs additional elements in case you need to apply structures which is not already present in the HTML. To layout something as a grid you need two levels of grouping: what delimits a cell and what delimits a row. Free-from dt/dd sequences does not contain this information, since there may be multiple dt per dd and vice versa. If dt/dd could only appear in pairs (like in your example) you could say there is an implicit structure, but this is not necessarily the case.