I've given up to try and educate XML heads that XML is just a proper subset of SGML, just as HTML is originally, and mostly still, an SGML vocabulary. Idk what people are talking about in this thread (seems to be about each one's personal preferences and wildly speculative assumptions about backtracking when in reality both SGML and WHATWG are deterministic); while there is exactly one reference to WHATWG at this time.