It existed also in DTD (Document Type Definition) used with HTML 4 and below, and XML. Came fromn SGML too I guess.
Yes it did, and there are HTML 5.x DTDs for HTML versions newer than HTML 4.x ar [1], including post-HTML 5.2 review drafts until 2023; see notes at [2].
[1]: https://sgmljs.net/docs/html5.html
[2]: https://sgmljs.net/blog/blog2303.html
alt Hacker News
Yes it did, and there are HTML 5.x DTDs for HTML versions newer than HTML 4.x ar [1], including post-HTML 5.2 review drafts until 2023; see notes at [2].
[1]: https://sgmljs.net/docs/html5.html
[2]: https://sgmljs.net/blog/blog2303.html