All attributes have a simple type--no complex types!

You can refer to your own simple type:

<xs:attribute name="href" type="my:domainURI"/>

Or you can embed the type definition

<xs:attribute name="href">
<xs:simpleType>
  <xs:restriction base="xs:anyURI">
    <xs:pattern value="http:.*"/>
  </xs:resritction>
</xs:simpleType>
</xs:attribute>