My first job in software was as a software test development intern at a ~500 employee non-profit, in about 2008 when I was about 19 or 20 years old. Writing software to test software. One of my tasks during the 2 years I worked there was to write documentation for their XML test data format. The test data was written in XML documents, then run through a test runner for validation. I somehow found out about XSLT and it seemed like the perfect solution. So I wrote up XML schemas for the XML test data, in XSD of course. The documentation lived in the schema, alongside the type definitions. Then I wrote an XSLT document, to take in those XML schemas and output HTML pages, which is also basically XML.
So in effect what I wrote was an XML program, which took XML as input, and outputted XML, all entirely in the browser at document-view time.
And it actually worked and I felt super proud of it. I definitely remember it worked in our official browser (Internet Explorer 7, natch). I recall testing it in my preferred browser, Firefox (version 3, check out that new AwesomeBar, baby), and I think I got it working there, too, with some effort.
I always wonder what happened with that XML nightmare I created. I wonder if anyone ever actually used it or maybe even maintained it for some time. I guess it most likely just got thrown away wholesale during an inevitable rewrite. But I still think fondly back on that XSLT "program" even today.