@doriantaylor I have a strong memory of setting my (xhtml) personal site to be served as XML with the right headers and being pleasantly surprised when syntax errors in markup showed up as XML parser failures rather than the browser doing its normal “best guess” thing and rendering a janky page. Felt like bringing compiled language syntax discipline to the web, and at the time that seemed like progress to me.
I also think that is why xhtml ultimately lost to html 4 and 5 though; I can’t imagine trying to run a CMS or any other site heavily focused on user generated content if the whole thing has to be valid as XML at all times. I’ve also come to think not everything needs to have that level of syntax rigor, and the democratic nature of the web is (was?) one of its best qualities