diff --git a/resources/schema/structures.xsd b/resources/schema/structures.xsd
index 73deaf9..34dc39d 100644
--- a/resources/schema/structures.xsd
+++ b/resources/schema/structures.xsd
@@ -197,6 +197,13 @@
         </xsd:restriction>
       </xsd:simpleType>
     </xsd:attribute>
+    <xsd:attribute name="format">
+      <xsd:simpleType>
+        <xsd:restriction base="xsd:string">
+          <xsd:enumeration value="lowercase"/>
+        </xsd:restriction>
+      </xsd:simpleType>
+    </xsd:attribute>
   </xsd:complexType>
 
   <xsd:complexType name="componentsComponentType">