The optional 
DeclHandler interface covers those aspects of DTDs
only a validating parser cares about:
Element declarations
Attribute declarations
Internal entity declarations
External entity declarations
An optional extension that not all parsers (particularly non-validating parsers) support
To set the DeclHandler for a parser, 
set the 
"http://xml.org/sax/handlers/DeclHandler" property. 
A SAXNotRecognizedException or SAXNotSupportedException will be thrown if the parser
doesn't support DeclHandler
One of the least useful interfaces in SAX2