RE: HREFs in XSL

[SNedunuri@pav.com]

Does anyone know where the official specification of a DTD can be found?
I've done a search at w3.org and at oasis and have turned up nothing. In
particular, I read somewhere that rule for !Element permits the & character
between element names to indicate that the elements may occur in any order
eg. <!Element Tag (Foo, Bar)> requires Bar to follow Foo
OTOH <!Element Tag (Foo & Bar)> would permit Foo and Bar to be in any order

But none of the unofficial guides to DTDs seem to mention this operator.

thanks