There are certain rules that apply to element type specializations.
A specialized element type has the following characteristics:
- A properly-formed class attribute that specifies the specialization hierarchy of the element
- A content model that is the same or less inclusive than that of the element from which it was specialized
- A set of attributes that are the same or a subset of those of the element from which it was specialized
- Values or value ranges of attributes that are the same or a subset of those of the element from which it was specialized
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.