Web Design in a Nutshell, 3rd Edition

Attlist (attribute) declarations

ATTLIST (attribute) declarations are used to declare the attributes permitted for a particular element. The following attribute declaration from the previous XHTML example says that the meta element may use the attributes id, http-equiv, name, content, and scheme. %i18n is an entity that represents still more available attributes (more on entities next).

<!ATTLIST meta
  %i18n;
  id          ID             #IMPLIED
  http-equiv  CDATA          #IMPLIED
  name        CDATA          #IMPLIED
  content     CDATA          #REQUIRED
  scheme      CDATA          #IMPLIED
  >

After each attribute name is its attribute type, which provides an indication of the type of information its value may contain. The most common attribute types are CDATA (character data) and an enumerated list of possible values (for example (left|right|center)). Other attribute types include ID, IDREF, IDREFS, NMTOKEN, NMTOKENS, ENTITY, ENTITIES, NOTATION, and xml: (a predefined XML value).

Finally, a default value is provided for each attribute. The default value itself may be listed, or there may be an indication of whether the attribute is required within the element (#REQUIRED), optional (#IMPLIED), or fixed (#FIXED value).