I wrote many thousands of lines of XSLT back in the day, converting XML into XSL:FO and pumping it through FOP. It worked remarkably well. It was my second experience with a declarative language (SQL being the first).
There were some great resources - libraries of code fragments, similar in spirit to tailwind ui - and people did crazy things with them.
We eventually abandoned it, after many years. It was indeed terribly difficult to work out how a document got transformed, and when returning to a transform after a long time working in other languages, you could easily spend a day just trying to work out how some trivial thing worked.
What XSLT really needed was a gui where you could play with the template and the input and see the output change in real time, a little like how those regex websites work. With a few colours and arrows to elucidate what bit was doing what. Not a trivial undertaking, granted. I wonder if such a thing was ever produced?
I do remember xsltproc being a game changer. it was still a CLI but it was really fast. So at least you could turn things around quickly. And IIRC Preview on the Mac would reload a PDF automatically if it changed. So you could get pretty close to a gui flow sometimes… as long as your document was short!
Until xsltproc, I’d been using whatever XSLT processor that came with Java, which (as always) was fine on a warmed up production server but sucked for REPL.
But what a gui could have helped with is working back from the output element to the node that generated it. That would have been sweet. But if memory serves anything commercial for XSLT back then was “enterprise licensing”, which we couldn’t afford.
I think the true lesson of xslt, is xml makes a terrible syntax for a programming language. The language itself minus the syntax is fine, although i think many people prefer a more imperative approach.
XSLT was also a terrible combination of two principles: Imperative coding + data pattern matching. For a novice beginner - and me at the time - I was never clear which pattern to use and how it best fit together.
XSLT 3 is a different beast, and XSLT 4 is being worked on at the moment.