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>