Comment by dragonwriter

> There was an attempt to nail its syntax down in the pre-HTML 5 days; that's XHTML. When HTML 5 pivoted away from that, that spelled the end of these two things ever coming together.

Exactly the opposite; WHATWG “Living Standard” HTML (different releases of which were used as the basis for W3C HTML5, 5.1, and 5.2 before the W3C stopped doing that) includes an XML serialization as part of the spec, so now the HTML-in-XML is permanently in sync with and feature-matched with plain HTML.