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.