<!DOCTYPE my-root [
                      <!ELEMENT my-root  (a*)>
                      <!ELEMENT a  EMPTY>
                      <!ATTLIST a
                          color  (red | green | blue | white) "white">
                  ] >
<my-root>

    <a color="red"/>        <!-- OK. -->
    <a color="black"/>     <!-- Invalid! -->

</my-root>