Validating a Custom DTD
http://www.alistapart.com/articles/customdtd/

Eisenberg, J. David
List Apart, A
2009

Abstract:

In his article in this issue, Peter-Paul Koch proposes adding custom attributes to form elements to allow triggers for specialized behaviors. The W3C validator won’t validate a document with these attributes, as they aren’t part of the XHTML specification. This article will show you how to create a custom DTD that will add those custom attributes, and will show you how to validate documents that use those new attributes.