Rules of specialization

You cannot make the content models of specialized element types less restrictive than the content models of their base types.

When you specialize one element from another, the new element must obey certain rules to be valid.

• A specialized element must have a content model equivalent to or more restrictive than its base element.

• A specialized element must have attributes that are equivalent to or a subset of the attributes of its base element.

• The attributes of a specialized element must have values or value ranges that are equivalent to or a subset of the values or value ranges of the base element.

• A specialized element must have a properly formed class attribute.

• Avoid overspecialized elements and crude specializations. Don’t create specialized elements when existing elements suffice.