XSL

From APIDesign

Extensible Stylesheet Language is language for transforming XML documents to another XML documents. The common rumor is that XSL code can be written, but cannot be read. As a result you can write a transformation, but later you cannot fix any bug in it (without causing tons of others) and it is better to start from scratch and write another XSL.

There is some truth on the above claim, but with help of proper unit testing, this can be neutralized. For example I have successfully used XSL to transform TheAPIBook from Docbook to OpenDocument.