I helped maintain (and later started rewriting) a medical web platform that used XSLT for it's HTML exactly like you described. It was powerful for sure, but a nightmare to maintain, extend or add to.

This in house framework was written before the advent of major web frameworks (at least in C# at the time) so I can understand where it came from. In fact, I still consider it an impressive feat of engineering. But XSLT to this day makes me shudder..