<!--non-validating processors may pass this instance because they don't check the IDREFS attribute type-->
<!DOCTYPE doc
[
<!ELEMENT doc (a|refs)*>
<!ELEMENT a EMPTY>
<!ELEMENT refs EMPTY>
<!ATTLIST refs refs IDREFS #REQUIRED>
<!ATTLIST a id ID #REQUIRED>
]>
<doc>
<a id="A1"/><a id="A2"/><a id="A3"/>
<refs refs=""/>
</doc>