Maybe it was a "viewer" in the 90s. The viewer is not a viewer - it is a full fledged application runtime that has a developer environment and media stack, along with several miscellaneous runtimes. A standard template language and document inclusion feature is very small peanuts compared to that. A teeny house compared to the galaxy already built-in - with several planets worth of features being added yearly.

You both make good points, and I come down on the side of adding some template mechanism to web standards. Of course, that all starts with an RFC and a reference implementation. Any volunteers?

Would raise my hand to volunteer for the reference implementation. I guess it would need to be in C++/Rust ? RFC, however, involves way too much talking and also needs solid networking amongst the web crowd. Not qualified for that. For a template language, it would be better to copy a subset from an existing de-facto standard like jinja2 which already has a lean, performant subset implementation at https://github.com/Keats/tera.

Document/template inclusion model should be OK now in modern era thanks to HTTP/3. Not really sure how that should ideally look like though.