[[wikipedia:Extensible_Stylesheet_Language|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]].