XSLT is best known as a "styling language", XML in one end, XML or HTML out the other.

Driven by data - push model like AWK - probably unfamiliar

Functional - like Lisp, Scheme, etc - probably unfamiliar

But is a Turing complete language that you can do real work in

So a stylesheet is actually a program. And programs need tests (not going to argue that point :))

BT Serviceview is an example of XSLT as a programming language