XSLT is a declarative language for transforming XML to other formats. XML is a pretty poor solution for configuration files (which is what everybody seemed to use it for back in the day) but works great for, you guessed it, marking up documents. If you have an XML schema for representing documents semantically you can then write an XSLT that quickly and efficiently converts it to, say, XHTML. This way you can work in a nice intermediate representation rich with meaning and specify the presentation as a function thereof.

It's not particularly sexy technology but it works well when used judiciously. Part of me wonders if XML will make some bizarre comeback in the era of LLMs for training data annotation or something similarly niche.

In combination with https://docs.hyperclay.com/docs/docs-tldr-paste-in-llm/ it could be used to build a lightweight offline configuration editor, for instance.

XSLT are the "XSL Transformations".

As described in [0]

> XSLT ... is a language for transforming XML documents into other XML documents. ... XSLT is designed for use as part of XSL, which is a stylesheet language for XML.

If you don't know what "XML" is, go ask your favorite search engine.

If you want to know who uses XSLT, go review the existing discussion on the topic. In summary, it's a bunch of people that Google doesn't really care about (and almost certainly far more people than use WebUSB or WebMIDI [1] and the like).

[0] <https://www.w3.org/TR/1999/PR-xslt-19991008>

[1] Yes, really.

I always thought of it as a specialized XML extension that describes transformations on other XML documents. Never used it, not sure why anything on my system uses it if I don't come across it in my daily software development life either but I have seen it as a dependency more times than I can remember.