I think the best examples of HTML in that regard is HTML-rendered info pages[0], for Emacs and its ecosystem. Then you have the same content presented in HTML [1]. Templates were enough in the first case. Includes are better in the second case due to common assets external to the content.

[0]: https://www.gnu.org/software/emacs/manual/html_node/emacs/in...

[1]: https://emacsdocs.org/docs/emacs/The-Emacs-Editor