In fact, I used them too around 2001 or so for my website when I did not enough programming/scripting to add a common layout to all content pages. That's how I originally implemented a two column layout: a narrow frame on the left for navigation links and a wide one on the right for content. I don't do it anymore, of course. I do know how to program now and generate my HTML website using a tiny Common Lisp program that adds common headers and footers to all my web pages.
We've come so far. Using a language from the 60s to write markup in a language from the 90s all so we can feel good by avoiding writing imperative code in another language from the 90s.
Seriously though, why can HTML not have a client side include?
In fact, I used them too around 2001 or so for my website when I did not enough programming/scripting to add a common layout to all content pages. That's how I originally implemented a two column layout: a narrow frame on the left for navigation links and a wide one on the right for content. I don't do it anymore, of course. I do know how to program now and generate my HTML website using a tiny Common Lisp program that adds common headers and footers to all my web pages.