Steps to reproduce:
1) Open https://github.com/jboss/metadata/blob/master/common/src/main/resources/schema/jbxb_1_0.xsd
2) Copy the text of the xsd
3) Go to http://www.utilities-online.info/xsdvalidation/
4) Paste the xsd into the text box on the right
5) Click the button under the text box on the right
6) Note the errors

Not valid.
Error - Line 94, 47: org.xml.sax.SAXParseException; lineNumber: 94; columnNumber: 47; src-element.3: Element 'class' has both a 'type' attribute and a 'anonymous type' child. Only one of these is allowed for an element.
Error - Line 19, 77: org.xml.sax.SAXParseException; lineNumber: 19; columnNumber: 77; src-resolve.4.2: Error resolving component 'jbxb:package'. It was detected that 'jbxb:package' is in namespace 'http://www.jboss.org/xml/ns/jbxb', but components from this namespace are not referenceable from schema document 'null'. If this is the incorrect namespace, perhaps the prefix of 'jbxb:package' needs to be changed. If this is the correct namespace, then an appropriate 'import' tag should be added to 'null'.
Error - Line 19, 77: org.xml.sax.SAXParseException; lineNumber: 19; columnNumber: 77; src-resolve: Cannot resolve the name 'jbxb:package' to a 'element declaration' component.