A note to my future self:
Scenario: validating a document using Nokogiri and a xsd which references other schemas.
Error: Something like:
schema.rb:37:in `from_document': attribute use (unknown), attribute 'ref': The QName value '{http://url.of.the.referenced.schema}blah' does not resolve to a(n) attribute declaration. (Nokogiri::XML::SyntaxError)
or the referenced schemas are using a URL to somewhere out there and Nokogiri tries to download it, in which case the script is slowed down or it throws a download error.

