<!DOCTYPE html>
<style>
rect {
  fill: green;
}
</style>
<svg width="50" height="50">
  <use xlink:href="#main"></use>
</svg>
<svg id="main" width="100" height="100">
  <g>
    <rect x="0" y="0" width="100" height="100"></rect>
  </g>
</svg>