I follow the principle that you should test everything that could possibly fail. And I would like to have unit tests for everything; tests that exercise each small behavior in isolation. The components that are easiest to unit test are typically class and methods in plain object-oriented code. Not everything falls into that category, though. Page templates, XSLT and other XML files are some counterexamples.
As I mentioned in my blog post on Paparrazzi testing, Uncle Bob (Robert C. Martin) has discussed how to test web templates or server pages. Since I'm currently working with Smarty templates, I wanted a simple way to run tests on them without needing to deal with a web server and the page navigation in a full web application.
Simpletest's web tester has a simplified web browser that's capable of running HTTP requests to test an actual running web application. To test a Smarty template in isolation, we can replace it with something even simpler that talks directly to the Smarty
