Not sure what you're after exactly but DTDs were introduced with SGML (ISO 8879:1986 [0]) and then used in simplified form with XML (which is specified as a simplified profile of SGML [1]).
The (historic) SGML-DTDs for HTML, including those used by W3C's validator and early IETF DTDs for HTML 2.0, can be found at W3C's site eg [2], [3].
Not really. SGML (and XML) are "meta-markup languages", meaning you declare your vocabulary yourself or use a ready-made one. There is in fact a simple general-purpose vocabulary declared in an ISO/IEC 8879:1986 appendix consisting of generic paragraph and heading elements, but it's not widely used in that form.
The (historic) SGML-DTDs for HTML, including those used by W3C's validator and early IETF DTDs for HTML 2.0, can be found at W3C's site eg [2], [3].
[0]: https://www.iso.org/standard/16387.html
[1]: https://www.w3.org/TR/REC-xml/
[2]: https://www.w3.org/TR/html4/sgml/dtd.html
[3]: https://www.w3.org/TR/2018/SPSD-html32-20180315/