Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A valid XML document is defined by the W3C as a well-formed XML document which also conforms to the rules of a Document Type Definition (DTD) or an XML Schema (XSD), which W3C supports as an alternate to DTD. This term should not be confused with a well-formed XML document, which is defined as an XML document that has correct XML syntax according to W3C standards.