How to validate a "ref" using XML::LibXML and/or xmllint

Hello,

I'm trying to validate that ref attributes reference existing elements. For
example:

<?xml version="1.0" encoding="utf-8" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
               elementFormDefault="qualified">
  <xs:element name="configuration">
    <xs:complexType>
      <xs:sequence>
        <xs:element minOccurs="0" maxOccurs="1" ref="interface" />
        <xs:element minOccurs="0" maxOccurs="1" ref="protocols" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="interfaces" type="xs:string"/>
  <xs:element name="protocols" type="xs:string"/>
</xs:schema>

Fails validation with XMLSpy with the error (since I missed the "s" in
interfaces):

'interface' must refer to an existing document

Both xmllint and XML::LibXML seem to ignore this error. I guess
is not hard to write Perl code to do that it using XML::XPath and friends
but
am I missing something here? Maybe some kind of "strict" mode that would
allow
this validation to occur?

Thanks,

Paulo

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