When you specialize one element from another the new element must
obey certain rules in order to be a valid specialization.
- The new element must have a content model that is equivalent to or more
restrictive than its parent.
- The new element must have attributes that are equivalent to or a subset
of the attributes of its parent.
- The new element's attributes must have values or value ranges that are
equivalent to or a subset of the parent's attributes' values or value ranges.
- The new element must have a properly formed class attribute.