We show that an XML DTD (Document Type Definition) can be viewed as the fixed point of a parametric content model. We then use natural transformations from the source content model to the target content model to derive DTDaware and validated XML document transformations. Benefits of such transformations include static type-checking of XML transformational programs, automatic validation of target documents, and modular compositions of XML document transformers. We prototype these modular XML document transformations in Objective Caml. The prototype depends heavily on the parametric module system of Objective Caml and is highly modular. Using Objective Caml to model XML document transformation also allows one to access high-level language constructs and supporting libraries in Objective Caml, hence enhance one’s productivity in XML programming. Categories and Subject Descriptors F.3.1 [Specifying and Verifying and Reasoning about Programs]: Specification Techniques. General Terms ...