Pretty much all of the HTML validator "errors" warnings for outdated attributes and the like. The "No space between attributes" one is pretty much the only real error.
Pointing the validator at this submission and filtering out obsolete elements/attributes and all warnings/info messages, I still got 221 errors: No DOCTYPE, an script element after closing the body, duplicate element ids, and invalid attributes (not obsolete, they must be be using them like data-*).