I remember I was enjoying XSLT; I used it to create templates and, in the end, valid XHTML pages. I thought that's the future.
At some point I made a news-articles site and I took that approach rather than databases: the owner would write XML files for each article that would be pretty simple: e.g. with tags like `title`, `image` (left, right, hero)`, some asides and some basic formatting for the body of the article, and XSLT, templates and CSS would make it a proper page. All the magic happened in the browser. I still think that's pretty cool.
That's pretty much what it was designed to do. You could push it pretty far too. Relational databases at the time all implemented XML types, so you could have XML all the way down, and XSLT is how you'd go from point A to point B.
But in the end, simpler alternatives prevailed.
JSON Schema takes 15 minutes to learn and it's a heck of a lot better than writing my own validator.