Validates the document against the schema, e.g., a DTD or an W3C XML schema or another. Any attempt to modify any part of the document while validating results in implementation-dependent behavior. In addition, the validation operation itself cannot modify the document, e.g., for default attributes. This method makes use of the error handler, as described in the [DOM Level 3 Core] DOMConfiguration interface, with all errors being SEVERITY_ERROR as defined in the DOMError interface.