Element Attributes

The final piece of the DTD puzzle involves attributes. You know attributes: they are the name/value pairs included with tags in your documents that control the behavior and appearance of those tags. To define attributes and their allowed values within an XML DTD, use the <!ATTLIST> directive:

<!ATTLIST element 
            attributes>

The element is the name of the element to which the attributes apply. The attributes are a list of attribute declarations for the element. Each attribute declaration in this list consists of an attribute name, its type, and its default value, if any.

Attribute Values

Attribute values can be of several types, each denoted in an attribute definition with one of the following keywords:

  • CDATA indicates that the attribute value is a character or string of characters. This is the attribute type you would use to specify URLs or other arbitrary user data. For example, the src attribute of the <img> tag in HTML has a value of CDATA.

  • ID indicates that the attribute value is a unique identifier within the scope of the document. This attribute type is used with an attribute, such as the HTML id attribute, whose value defines an ID within the document, as discussed in the Section B.1 in Appendix B.

  • IDREF or IDREFS indicates that the attribute accepts an ID defined elsewhere in the document via an attribute of type ID. The ID type is used when defining IDs; IDREF and IDREFS are used when referencing a single ID and a list of IDs, respectively.

  • ENTITY or ENTITIES ...

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