I posted to xmlschema-dev about this but didn't get any feedback, so I'm opening this.

Consider this schema:

<xs:schema ...>
    <xs:defaultOpenContent mode="interleave">
       <xs:any namespace="xyz:myURI">
    </xs:defaultOpenContent>
    ...
    <xs:complexType name="Base">
       <xs:openContent mode="interleave">
          <xs:any namespace="##any">
       </xs:openContent>
       <xs:sequence>
          <xs:element name="elemOne"/>
       </xs:sequence>
    </xs:complexType>

    <xs:complexType name="Derived">
       <xs:complexContent>
          <xs:extension base="Base">
               <xs:sequence>
                   <xs:element name="elemTwo"/>
                </xs:sequence>
          </xs:extension>
       </xs:complexContent>
    </xs:complexType>
   
    ...

</xs:schema>

In Base, we use an open content that allows elements from any namespace, whereas the default the open content is more restrictive (only match xyz:myURI).  We extend Base to create Derived.  According to  Mapping Rules for Content Type Property of Complex Content (§3.4.2.3.3), Derived gets its {open content} from the schema default, allowing only xyz:myURI.  However, that violates 3.4.6.2 Derivation Valid (Extension) (point 1.4.3.2.2.4 - the namespace subset requirement).

When applying step 4.2.3 in the mapping rules to Derived, it looks like we're getting ready to inherit the {open content} from Base, but then we turn around in steps 5 and 6 and take the default open content, which leads to the above violation.  I find it hard to believe that this was the intended behavior.  I would have thought it most natural to inherit the open content from the base type.  

It may be that I am missing something, or that what I just described is really the intended behavior, but I am certainly surprised by it, and suspect users of XSD would be likewise surprised.  It isn't entirely unreasonable behavior, but I found it surprising enough that I wanted to pose this question to make sure an error wasn't made in the mapping.

Thanks,

-- 
Kevin Braun
Objective Systems, Inc.
http://www.obj-sys.com