XML Element Attributes

To enforce the attribution properties of a tag, you must add an attribute-list-declaration to the document's DTD. This list accomplishes the following:

• Lists the names of all of the attributes associated with a specific element
• Specifies the data type of each attribute
• Indicates whether each attribute is required or optional
• Provides a default value for each attribute, if necessary

The general attribute syntax is:

<!ATTLIST element attribute1 type1 default1
                  attribute2 type2 default2
                  attribute3 type3 default3 ...>

Here element is the element name associated with the attribute, attribute is the attribute name, type is the attribute's data type, and default indicates whether the attribute is required and whether it has a default value. In practice, this declaration can be split into several single ones, each describing a specific attribute:

Attribute types

XML supports 9 attribute types, we consider only 4 of them here:

CDATA allows an attribute to contain almost any data except some reserved symbols like <, >, &.

Example:

<!ATTLIST item itemPrice CDATA>
allows the following values:
<item itemPrice="29.95">

In some cases you'll want to restrict the set of possible attribute values. You can do it with the following syntax:

Example:

<!ATTLIST customer custType (home | business) ... >

An ID token is used whenever the attribute value must be unique in the document. Ones an ID is defined, other attribute values can reference to it by using the IDREF token.

Attribute defaults

There are 4 possible values listed in the following table:

Examples:

<!ATTLIST img alt CDATA #REQUIRED>
<!ATTLIST html xmlns:mod CDATA #IMPLIED>
<!ATTLIST customer custType (home | business) "home">
<!ATTLIST frame noresize CDATA #FIXED "noresize">