es a convenient way of doing
 this.

 <p>
 The current version of the specification only provides one formal way of indicating the observable type of a tag
 handler: its tag handler implementation class, described in the tag-class subelement of the tag element. This is
 extended in an informal manner by allowing the tag library author to indicate in the description subelement an
 observable type. The type should be a subtype of the tag handler implementation class or void. This addititional
 constraint can 