By itself (without CSS or JS), HTML (5 or otherwise) cannot possibly be Turing-complete because it is not a machine. Asking whether it is or not is essentially equivalent to asking whether an apple or an orange is Turing complete, or to take a more relevant example, a book.
HTML is not something that “runs”. It is a representation. It is a format. It is an information encoding. Not being a machine, it cannot compute anything on its own, at the level of Turing completeness or any other level.