a way of validating xml; assertion-based; familiar to TDD-ers.

from wikipedia: "...Schematron is a rule-based validation language for making assertions about the presence or absence of patterns in XML trees. It is a structural schema language expressed in XML using a small number of elements and XPath."

wonderful introduction (especially the section 'Schematron hierarchy'), and the full article includes information about how schematron complements other xml schemas

• 'assertions' define conditions to test
• 'rules' define the context, or where in the document to apply the conditions

hands on tutorial, ibm developerworks

concise schematron test examples, showing, for each example, the schematron test xml, minimalist source xml that passes and fails and expected test output.

schematron validation with lxml, a popular python xml library