@chriscoyier asks a good question: “Why can’t HTML alone do includes?”
https://frontendmasters.com/blog/seeking-an-answer-why-cant-html-alone-do-includes/
Pure-HTML includes would help with a static site generation issue:
• Whenever you change the (cross-page) user interface, you have to regenerate the whole blog.
• If the shared bits were stored in separate files, only those would have to be regenerated.