<svg xmlns="http://www.w3.org/2000/svg" 
     xmlns:xlink="http://www.w3.org/1999/xlink">
<defs>
  <clipPath id="myclip">
    <rect x="0" y="0" width="200" height="200" style="transform: scale(.5)"/>
    <!-- second rect causes masking -->
    <rect x="0" y="0" width="200" height="200" style="transform: scale(.5)"/>
  </clipPath>
</defs>
<g  clip-path="url(#myclip)">
  <rect x="0" y="0" width="200" height="200" fill="red"/>
  <rect x="0" y="0" width="100" height="100" fill="green"/>
</g>
</svg>