There's no lack of syntactical templating mechanisms when HTML is hosted within a SGML markup processing context (ie how HTML started life), and no need to add

There's no lack of syntactical templating mechanisms when HTML is hosted within a SGML markup processing context (ie how HTML started life), and no need to add templating at the HTML markup vocabulary level either: For server-side rendering, SGML (and also some other template "engines") provide HTML-aware, type-checked, parametric macro expansion. Actually, SGML templating works transparently on the client side and the server side. Within the browser OTOH, there's already JavaScript, making every dynamic feature added to HTML inessential for better or worse, like it has for over 25 years now. That's just how it was decided a long time ago, and adding half-assed features to HTML like the template element (which requires JavaScript, in turn, thus doesn't add any essential capability) all the time is exactly the thing we shouldn't be doing when the damn "web stack" is already bloated as fuck.