Hi ! The ICal DTD (P32) does not allow an Extref element to have a fmttype attribute. According to RFC 2445, P77, an attach property can have a fmttypeparam even if her value type is URI. So I suggest to replace P32 : <!ATTLIST extref uri ENTITY #REQUIRED> by : <!ATTLIST extref %attr.fmttype; uri ENTITY #REQUIRED> Please correct me if I m wrong. Best Regards Philippe Lesage