> I remember back in the 2000s writing purely XML websites with stylesheets for display

Awesome! I made a blog using XML+XSLT, back in high school. It was worth it just to see the flabbergasted look on my friends faces when I told them to view the source code of the page, and it was just XML with no visible HTML or CSS[0].

[0] https://www.w3schools.com/xml/simplexsl.xml - example XML+XSLT page from w3schools