You could do some awesome things with XML/XSL. My favorite was RSS feeds or OPML collections of RSS feeds (both flavors of XML) calling an XSL transformation when viewed in a web browser instead of mined by computers for data; same with TEI files, although that was a bit more arcane. The idea was that you'd have one data file for machine use and a way to present that data to humans as a web page.

But, as a stack, the XML/XSL/XPath/etc combination was weak because XSLT was hard to comprehend and never updated. No browser supports XSLT 2 or 3; they froze support with 1. XSLT 2 was 2007, a year before HTML 5 was a thing, yet it never got picked up by browsers. Javascript was just easier to understand than XSLT. Development of XHTML was also glacial, and people wanted new tags, especially for video.