1 <!--non-validating processors may pass this instance because they don't check the IDREFS attribute type-->

     2 <!DOCTYPE doc

     3 [

     4 <!ELEMENT doc (a|refs)*>

     5 <!ELEMENT a EMPTY>

     6 <!ELEMENT refs EMPTY>

     7 <!ATTLIST refs refs IDREFS #REQUIRED>

     8 <!ATTLIST a id ID #REQUIRED>

     9 ]>

    10 <doc>

    11 <a id="A1"/><a id="A2"/><a id="A3"/>

    12 <refs refs=""/>

    13 </doc>