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
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 can be of several types, each denoted in an attribute definition with one of the following keywords:
CDATAindicates 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
srcattribute of the
<img>tag in HTML has a value of
IDindicates 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
idattribute, whose value defines an ID within the document, as discussed in the Section B.1 in Appendix B.
IDREFSindicates that the attribute accepts an ID defined elsewhere in the document via an attribute of type
IDtype is used when defining IDs;
IDREFSare used when referencing a single ID and a list of IDs, respectively.