Specialization rules for element types

There are certain rules that apply to element type specializations.

Characteristics

A specialized element type has the following characteristics:

• It has a properly-formed @class attribute that specifies the specialization hierarchy of the element
• It has a content model that is the same or less inclusive than that of the element from which it was specialized. When using expansion modules to add a new element to a content model, the generalized form of any new element is also allowed in the original content model.
• It has a set of attributes that are the same or a subset of those of the element from which it was specialized, except for specializations of @base or @props
• The values or value ranges for each attribute are the same or a subset of those attributes on the element from which it was specialized.

Namespaces

DITA elements are never in a namespace. Only the @DITAArchVersion attribute is in a DITA-defined namespace. All other attributes, except for those defined by the XML standard, are in no namespace.

This limitation is imposed by the details of the @class attribute syntax, which makes it impractical to have namespace-qualified names for either vocabulary modules or individual element types or attributes. Elements included as descendants of the DITA <foreign> element type can be in any namespace.