This interface represents an XML attribute information item. It can be initialized
with the string value of an attribute and the implementation must convert the string into
an object of the appropriate type. The getContent() method will return the converted
Object and the caller must cast this to the appropriate type.
If a conversion error occured because the string was not in the correct form,
the isValid() method will return false. The toExternalForm() method will return the
attribute's original string value.