the process of checking to see if a xml document conforms to a schema is called validation, which is separate from xml's core concept of syntactic well-formedness. all xml documents must be well-formed, but it is not required that a document be valid unless the xml parser is "validating", in which case the document is also checked for conformance with its associated schema. dtd-validating parsers are most common, but some support xml schema or relax ng as well.