mono-xmltool --validate-dtd https://raw.githubusercontent.com/gentoo/dotnet/master/dev-dotnet/referenceassemblies-pcl/metadata.xml
The element 'pkgmetadata' has invalid child element 'herd'. List of possible elements expected: 'maintainer natural-name longdescription slots use upstream'.
XSD schema itself is XML too, therefore it can be validated against the W3 DTD like this:
wget -c http://www.gentoo.org/dtd/metadata.dtd
xmllint --noout --dtdvalid http://www.w3.org/2001/XMLSchema.dtd metadata.xsd
