2.1.8 [W3C-XSS] Section 4.2.3, References to schema components across namespaces


The specification states:

 Note: Since both the namespace and schemaLocation [attribute] are optional, 
 a bare <import/> information item is allowed. This simply allows unqualified 
 reference to foreign components with no target namespace without giving 
 any hints as to where to find them.


An unqualified reference results in an invalid schema document.