You make this more complex than it actually is. HTML is basically SGML with a DTD declaring rules for tag omission/inference, empty elements, and attribute shor

You make this more complex than it actually is. HTML is basically SGML with a DTD declaring rules for tag omission/inference, empty elements, and attribute shortforms. This is what the majority of XML heads seem to struggle with, but which nevertheless is every bit as formal as XML is, XML being just a proper subset of SGML, though too small for parsing HTML. Then HTML has a number of historical quirks including: - the style and script elements have special rules for comments that would make older pre-CSS, pre-Javascript browsers just see markup comments and ignore those - the URL syntax using & ampersand characters needs to be treated special because & starts an entity reference in SGML's default concrete syntax - the HTML 5 spec added a procedural parsing algorithm (in addition to specifying a grammar-based spec) that basically parses every byte sequence as HTML via fallback rules; for most intents and purposes, the language recognized by these rules, taken to the extreme, is not what's commonly understood as HTML - WHATWG have added a number of element content rules on top of the HTML 4.01/HTML 5.0 baseline with ill-defined parsing rules (such as the content models for tables and description lists); the reason is precisely that WHATWG, once Ian Hickson had distilled the HTML DTD 4.01 grammar rules into the HTML 5 grammar presentation as prose, a formal basis was no longer used for vocabulary extension