XSL (Extensible Stylesheet Language) the stylesheet part, it included XSLT (transform), XPath, and XSL-FO (XSL Format Objects).

XSLT and XPath each went on to great solo careers, XSL-FO kind of fizzled when XML+XSL failed to displace HTML+CSS.

XSL:FO is great for turning into PDFs though.

There's also XQuery for transformations and XSD/RelaxNG for schemas, the whole combination of XML database, WYSIWYG (via XSLT) XML editor and output transformations is very powerful, every journal we published and every site we ran at the British Medical Journal was generated that way.

I miss XSLT every time I see megabytes of JavaScript converting some JSON into a soup of HTML to display in browsers. A bunch of JavaScript with a bunch of overwrought "components" is serving as a very complicated templating system.

A lot of really stupid JavaScript could be done fairly easily with XSLT natively inside a browser engine. An XML document can point to its own XSL(s) so get rendered to the appropriate delivery format or just used directly as data.

I don't miss it. I had several projects where it seemed like a good idea. I pushed it as a good idea. I really wanted it to work. Some developers (usually the sharper bulbs) got it. A lot of developers really struggled with it. Especially the folks who were 'front end' developers. Even in the 2000's, when everything was trying to go XML, staffing those roles was hard.

Tooling was XSLT's Achilles heel. Too many tools had no or poor XSLT support. What's confusing to me is a lot of the JavaScript templating today has equally bad tooling yet everyone's in love with it.

Doesn't that suggest that tooling wasn't the issue after all?

The lack of tooling for XSLT and modern JavaScript stuff are on different sides of the adoption curve. XSLT lacked tooling on the upside of the curve so it never hit critical mass. Its other problems could have been solved had it hit critical mass and seen wider adoption.