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Wasn't HTML4 the last one defined as a SGML DTD? 5 and on is its own beast.

(rip XHTML)

You are right. There is a third-party DTD that should be mostly compatible (https://sgmljs.sgml.net/docs/html5.html).

In reality, HTML4 was never implemented to the letter by user agents, because people do things like putting -- inside comments.