unit testing jsp 2.0 .tag files

How do people test jsp 2.0 tags? The ones that have .tag extensions and are basically jsp files with some extra support for passing parameters.

From stackoverflow

• The way we do this is by making a jsp page as a test fixture and running the asserts through selenium. Hardly a "quick" unit test, but not as inefficient as you could fear.

  You want that selenium test-rig up an running anyway, right ? We use it both for some unit-test like fixtures and to run JsUnit tests in a systematic manner.