<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<defs>
<!-- You should see two circles with different sizes -->
<mask id="mask" x="0%" y="0%" width="100%" height="100%" maskContentUnits="objectBoundingBox">
    <circle cx=".5" cy=".5" r=".5" fill="white"/>
</mask>
</defs>
<rect x="20" y="20" width="100" height="100" style="fill:green;mask:url(#mask);"/>
<rect x="140" y="50" width="100" height="40" style="fill:green;mask:url(#mask);"/>
</svg>