Is there a built-in schema datatype for xhtml data? Suppose I want to specify a "boozle" element that contains two "woozles", each of which is arbitrary xhtml. I want to write something like this, using the relax NG compact syntax:

namespace nifty = "http://brinckerhoff.org/nifty/"

woozle = element nifty:woozle {xhtml}

Unfortunately, xmllint then signals this error:

./lab.rng:43: element ref: Relax-NG parser error : Reference xhtml has no matching definition ./lab.rng:43: element ref: Relax-NG parser error : Internal found no define for ref xhtml

So my question is this: is there something sensible that I should put in place of "xhtml" above?