That's the first thing listed in the article? "Javascript to go fetch the HTML and insert it". What they're after is something that's _just_ HTML and not another language.
hunh - THAT's cool. Learn something new every day.
That said - it does appear encapsulated, much like an iframe - css doesn't take, etc. so though more "pure" HTML I'd argue doesn't fulfill the spirit of "include"?
While you do need a server i think this is the functional equivalent? The fetch JS and insert outlined (linked to) in the article is async. This blocks execution like you'd expect an HTML include to do. It's WAY easier to reason about - which is why the initial ask, I think...
you need a server for HTML to work, as practical matter. But yes. There IS a workaround to that too, if you're REALLY determined, but you have to format your HTML a giant JS comment block (lol really :))
[edit: I'm sure there are still some file:// workflows for docs - and yes this doesn't address that]