To enforce structure requirements for an XML document, you have to turn to one of XML's attendant technologies, data type definition (DTD). And that's the subject of this article.

A DTD is associated with an XML document by way of a document type declaration, which appears at the top the XML file (after the line).

XML allows designers to custom design document types. Whereas everyone developing in HTML is burdened with its all-inclusive nature, XML can be used to create any type of document. HTML must be all ...