In a DTD, attributes are declared with an ATTLIST declaration.
An attribute declaration has the following syntax:
The attribute-type can be one of the following:
|CDATA||The value is character data|
|(en1|en2|..)||The value must be one from an enumerated list|
|ID||The value is a unique id|
|IDREF||The value is the id of another element|
|IDREFS||The value is a list of other ids|
|NMTOKEN||The value is a valid XML name|
|NMTOKENS||The value is a list of valid XML names|
|ENTITY||The value is an entity|
|ENTITIES||The value is a list of entities|
|NOTATION||The value is a name of a notation|
|xml:||The value is a predefined xml value|
The attribute-value can be one of the following:
|value||The default value of the attribute|
|#REQUIRED||The attribute is required|
|#IMPLIED||The attribute is optional|
|#FIXED value||The attribute value is fixed|
In the example above, the "square" element is defined to be an empty element with a "width" attribute of type CDATA. If no width is specified, it has a default value of 0.
Use the #REQUIRED keyword if you don't have an option for a default value, but still want to force the attribute to be present.
Use the #IMPLIED keyword if you don't want to force the author to include an attribute, and you don't have an option for a default value.
Use the #FIXED keyword when you want an attribute to have a fixed value without allowing the author to change it. If an author includes another value, the XML parser will return an error.
Use enumerated attribute values when you want the attribute value to be one of a fixed set of legal values.