XSLT – Native, zero-config build system for the Web

XSLT was many people’s first foray into functional programming (usually unwilling, because their company got a Google Search Appliance or something). I can’t imagine ever reaching for it again personally, but it was useful and somewhat mind-expanding in its heyday.

I made many transformation pipelines with XSLT back in the days, and even a validation engine using Schematron; it was one of the most pleasant experience I had.

It never broke, ever.

It could have bugs, of course! -- but only "programmer bugs" (behavior coded in a certain way that should have been coded in another); it never suddenly stopped working for no reason like everything does nowadays.