Elements can be parsed from a document instance and a first validation of each parsed element can be performed using a first schema parse tree. The first schema parse tree can include nodes for respective types of document elements expected to be encountered and that correspond to schema elements parsed from at least one schema and validated using a second schema parse tree. At least one of the nodes can be configured to call at least one validation rule, external to the first schema parse tree, which validates the element of the document instance corresponding to the node. A second validation of at least one parsed element can be performed using the validation rule called from the node to which the parsed element corresponds. The method also can include adding the validated document elements to a validation report and outputting the validation report.