Unit testing is a Good Thing. An Essential Thing, even, for people who write computer programs. The idea is that you make sure that each little bit of the program is tested automatically to ensure that it does its job, and you keep running the tests as you add to the program. Helps catch the bugs that you create when changes have side effects on other parts of the program and makes it very easy to make major changes to a program. Other kinds of testing like getting users to use it to do stuff with it and simulating users doing stuff are still required.
A long time ago, I wrote about unit testing for XSLT as practiced Jacek Radajewski - back then he worked for USQ and I didn't. Now I do and he doesn't.
Anyway the system Jacek and the team at USQ developed has been released. If you do XSLT, and you can run Java (which means pretty much everyone if they persevere) then you should look at Jacek's UTF-X project.
There's a UTF-X page here (I think he wants java.net to be the home eventually):
There's a bit more to see on Sourceforge.
We use UTF-X to test all the XSLT code written for the ICE project we're working on at USQ. If you're doing XSLT you care about and you don't have some kind of unit test framework then you should use it too. If you use some other framework would you mind telling me what it is?
(Updated - I forgot to mention that UTF-X was a team effort)