If an XML document has an associated document type definition and if the document complies with the constraints expressed in it, it is valid. The document type definition must appear before the first element in the document. The name following the word DOCTYPE in the document type definition must match the name of root element.

A Document Type Definition provides applications with advance notice of what names and structures can be used in a particular document type.

If an element name in DTD is followed by

• the star [*] - This element can occur zero, once or several times.
• the plus [+] - This element can occur once or several times.
• the question mark [?] - This element can occur zero or one times.

To associate name-value pairs with elements attributes are used . Attribute specifications may appear only within start-tags and empty-element tags. The declaration starts with ATTLIST then follows the name of the element the attributes belong to and then follows the definition of the individual attributes.

No element type may have more than one ID attribute specified. The value of an ID attribute must be unique between all values of all ID attributes.

An element can be defined EMPTY - It can contain only attributes but no text.