XHTML elements in Schema

Hi,

I am writing an XML schema that is supposed to constraint the format of xml files that form a basis for web pages. However, there is one place where I would like to give people the chance to include html-code in the xml. How can I achieve this without having to rewrite the xhtml schema? And then, how do I use the resulting schema in XML?

Maybe I can even restrict what xhtml users can use (e.g. only what is allowed inside body?).

thanks
S