[niftich]

No. HTML is not formally recognized to be a 'Structured Syntax' of upon which semantically richer standalone mediatypes can be built [1]. This is because existing deployments favor a different approach of imbuing additional semantics inside HTML documents -- microformats -- which place the mechanism of understanding on an opportunistic parser, vs. a restrained one that only executes if its preferred mediatype is advertised. Appendix A of RFC 3023 [2] offers a thorough treatment of this matter. Not defining +html is essentially a concession that enables the two schools of thought to coexist side-by-side.

This is the same difference in schools that I express in a different comment [3] in this thread.

[1] https://tools.ietf.org/html/rfc6839 [2] https://tools.ietf.org/html/rfc3023#appendix-A [3] https://news.ycombinator.com/item?id=14361842