I'm using XSLT 2.0, trying to open an XML file with document(), the
XML file references a DTD that is not available, and document() fails.
How can I get document() to ignore the DTD?

How are you using XSLT 2? From Java? There is a parser property you can
set for Xerces to ignore external DTD of an XML. Or do you expect to do
that for just one particular document call?