Modular XHTML

I am looking for an XML/XHTML editor that facilitates the use of modular DTDs, and a parser that will work with Modular DTDs.

Does anyone have information on a product like this?

Or can anyone direct me to real world implementation of XHTML mods, or relatively solid information on mods other than the W3c?

Also does Xerces and Xalan work with modular XHTML?

Any info appreciated.

-- Margaret