How to ignore the validation of Unknown tags?

In case your not already done with this, you might try the following:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="root" type="root"></xs:element>
  <xs:complexType name="root">
    <xs:sequence>
      <xs:any maxOccurs="2" minOccurs="0" processContents="skip"/>
      <xs:element name="node" type="xs:string"/>
      <xs:any maxOccurs="2" minOccurs="0" processContents="skip"/>
    </xs:sequence>
  </xs:complexType>
</xs:schema>

Under Linux this works fine with xmllint using libxml version 20706.

You could make use of a new feature in XML 1.1 called "Open Content". In short, it allows you to specify that additional "unknown" elements can be added to a complex type in various positions, and what the parser should do if it hit any of those elements.

Using XML 1.1, your complex type would become:

<xs:element name="root" type="root" />
<xs:complexType name="root"> 
  <xs:openContent mode="interleave">
    <xs:any namespace="##any" processContents="skip"/>
  </xs:openContent>

  <xs:sequence> 
    <xs:element name="node" type="xs:string"/> 
  </xs:sequence> 
</xs:complexType>

If you have a lot of complex types, you can also set a "default" open content mode at the top of your schema:

<xs:schema ...>
  <xs:defaultOpenContent mode="interleave">
    <xs:any namespace="##any" processContents="skip"/>
  </xs:defaultOpenContent>

  ...
</xs:schema>

The W3C spec for Open Content can be found at http://www.w3.org/TR/xmlschema11-1/#oc, and there's a good writeup of this at http://www.ibm.com/developerworks/library/x-xml11pt3/#N102BA.

Unfortunately, .NET doesn't support XML 1.1 as of yet I can't find any free XML 1.1 processors - but a couple of paid-for options are:

• http://www.saxonica.com/
• http://www.altova.com/raptorxml.html

Conclusion:

This is not possible with XSD. All the approaches I was trying to achieve the requirement were named as "ambiguous" by validation-tools, accompanying bunch of errors.