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* * * In DTD, attributes are declared using the ATTLIST declaration. * * * ## Declaring Attributes Attribute declarations use the following syntax: DTD Example: XML Example: ```xml The following are the options for **attribute types**: | Type | Description | | --- | --- | | CDATA | The value is character data | | (_en1_|_en2_|..) | The value must be one of the 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 the name of a notation | | xml: | The value is a predefined XML value | Default **attribute values** can use the following values: | Value | Explanation | | --- | --- | | Value | The default value of the attribute | | #REQUIRED | The attribute value is required | | #IMPLIED | The attribute is not required | | #FIXED value | The attribute value is fixed | * * * ## Default Attribute Values DTD: Valid XML: ```xml In the above example, "square" is defined as an empty element with a "width" attribute of type CDATA. If the width is not specified, its default value is 0. * * * ## #REQUIRED ### Syntax ### Example DTD: Valid XML: ```xml Invalid XML: ```xml If you do not have a default value option but still want to require authors to provide the attribute, use the keyword #REQUIRED. * * * ## #IMPLIED ### Syntax ### Example DTD: Valid XML: ```xml Valid XML: ```xml If you do not want to force authors to include the attribute and you do not have a default value option, use the keyword #IMPLIED. * * * ## #FIXED ### Syntax ### Example DTD: Valid XML: ```xml Invalid XML: ```xml If you want the attribute to have a fixed value and do not allow authors to change it, use the keyword #FIXED. If the author uses a different value, the XML parser will return an error. * * * ## Enumerated Attribute Values ### Syntax ### Example DTD: XML Examples: ```xml or ```xml If you want the attribute value to be one of a set of fixed, valid values, use enumerated attribute values.