Your problem here is that you're hand-writing HTML including all the templates. This wasn't a good way to do it 30 years ago and it's not a good way to do it now.

See all these "static site generators" everyone's into these days? We used those in the mid-90s. They were called "Makefiles".

Yeah because I was 11 and didn't know what a Makefile was. That's my point. I wanted to make web pages, and didn't know any programming. HTML is designed to be hand-written. You just write text, and when you want it to look different, you wrap it in a thing. When doing this, you'll quickly want to re-use snippets/invent your own tags. XSLT gives a solution to this without saying "okay let's back up and go learn how to use a command line now, and probably use an entirely different document format" (SSGs) or "okay let's back up and learn about functions, variables, classes, and callbacks, and maybe a compiler" (Javascript). It just says "when you want to make your own tags, extract them into a 'template' tag, then include your templates just like you include a CSS file for styles".