I finally found (after I googled "libxml-ruby xsd") a "Recipe" how to
validate XML-files against DTD/XSD!
Here is the link: http://codeidol.com/other/rubyckbk/XML-and-HTML/Validating-an-XML-Document/
I think it's using "libxml-ruby", but (as I understood it) it is not a
"pure-Ruby"-solution because it's using a C library. But that's no
problem.

Thanks for your hints!

Best Regards,
- dominik aumayr