[susam]

Yes! Those were made with the now obsolete <frameset> tag. An example here: https://www.quackit.com/html/html_4/tags/html_frameset_tag.c...

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.

[rezonant]

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?

[lelanthran]

Thankfully you can do it now in a custom elements. I used one, I assumed everybody else would have one by now, too

[colanderman]

You can do client-side include with XSLT, which (surprisingly!) all major browsers support.