Developement of code

Hi All,
         I want  to develop a code validator according to my own rules.I have downloaded the Markup Validation Servicev0.7.2 and installed locally.I have downloaded complete set of DTD's.It is working fine for Doctype like XHTML,HTML etc...There is not much help available in help documents to modify the code.If i want to modify the code how i have to do it.Can anybody help me in this regard.
     
Thanks & Regards,
Suman