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.
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.