XHTML validation, custom namespaces and attributes

Question

I have a website that a client requires to be XHTML valid. They check it using the W3C validator.

I want to add my own namespace and attributes so I can do things li

Answer 1

1.0 not allowed this .if you really want to do this you should download existing dtd and save it in your server and define private instead of public. but w3c validation not passed because it validate against the orginal one only