XML in a Nutshell, 3rd Edition

Attribute Defaults

In addition to providing a data type, each ATTLIST declaration includes a default declaration for that attribute. There are four possibilities for this default:

#IMPLIED

The attribute is optional. Each instance of the element may or may not provide a value for the attribute. No default value is provided.

#REQUIRED

The attribute is required. Each instance of the element must provide a value for the attribute. No default value is provided.

#FIXED

The attribute value is constant and immutable. This attribute has the specified value regardless of whether the attribute is explicitly noted on an individual instance of the element. If it is included, though, it must have the specified value.

Literal

The actual default value is given as a quoted string.

For example, this ATTLIST declaration says that person elements can but do not need to have born and died attributes:

<!ATTLIST person born CDATA #IMPLIED
                 died CDATA #IMPLIED
>

This ATTLIST declaration says that every circle element must have center_x, center_y, and radius attributes:

<!ATTLIST circle center_x NMTOKEN #REQUIRED
                 center_y NMTOKEN #REQUIRED
                 radius   NMTOKEN #REQUIRED
>

This ATTLIST declaration says that every biography element has a version attribute and that the value of that attribute is 1.0, even if the start-tag of the element does not explicitly include a version attribute:

<!ATTLIST biography version CDATA #FIXED "1.0">

This ATTLIST declaration says that every web_page element has a protocol attribute. If a particular ...