<stc:STCResourceProfile
xmlns="http://www.ivoa.net/xml/STC/stc-v1.30.xsd">
<AstroCoordSystem xlink:type="simple"
xlink:href="ivo://STClib/CoordSys#UTC-FK5-TOPO"
/>
<AstroCoordArea
coord_system_href="ivo://STClib/CoordSys#UTC-FK5-TOPO" >
<AllSky />
</AstroCoordArea >
</stc:STCResourceProfile >
coord_system_href can be defined as an xlink:href in schema. You are only allowed one of these per element and that is necessary and sufficient.
Ed