<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<defs>
<mask id="mask">
    <rect x="0" y="0" width="140" height="140" fill="rgb(0%,90%,0%)"/>
    <circle cx="70" cy="70" r="40" fill="rgb(0%, 99%, 0%)"/>
</mask>
</defs>
<rect x="20" y="20" width="100" height="100" style="fill:black;mask:url(#mask)"/>

<!-- reference image - should be identical to above -->
<rect x="150" y="20" width="100" height="100" fill="rgb(111,111,111)"/>
<circle cx="200" cy="70" r="40" fill="rgb(76,76,76)"/>
</svg>