XSLV Static Validation Tool
The XSLV tool for static validation of XSLT from the University of Aarhus, available online at http://www.brics.dk/XSLV/, is able to check that all output of a stylesheet at runtime is valid according to a specified output schema, assuming that the input is valid according to its specified schema. Schemas can be written as DTD, XML Schema, or a subset of RELAX NG.
Since XSLT is Turing complete, determining validity for all possible stylesheets is undecidable, so the tool applies some approximations. However, the designers wanted to be able to guarantee correctness, so the approximations err on the safe side: the tool reports some valid output as being invalid, but should never report invalid output as valid.
I consider that this type of static validation complements rather than replaces unit testing, firstly since the stylesheet output is likely to be invalid from its inception up until it is largely complete, and secondly since its all too easy to create output that’s valid but still incorrect.