<svg width="800" height="600" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<style>
#d0 {
fill: red;
d: path('M 200 100 A 100 100 0 1 1 200 100');
}
#p0 {
d: inherit;
}
#g0 {
fill: green;
d: path('M 100 100 H 300 V 300 H 100 V 100');
}
#u0 {
d: inherit;
}
</style>
<defs id="d0">
<path id="p0" />
</defs>
<g id="g0">
<use id="u0" xlink:href="#p0" />
</g>
</svg>