If you want to have an attribute on an element with simple typed content (e.g. string), you have to "extend" that type:

<xs:element name="a">
   <xs:simpleContent>
      <xs:extension base="xs:string">
         <xs:attribute name="href" type="xs:anyURI"/>
      </xs:extension>
   </xs:simpleContent>
</xs:element>

This "extends" the simple type 'xs:string' to have an attribute.

Type extension will be talked about in the next lecture.