<?xml version="1.0" encoding="utf-8"?>

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">

<defs>
    <mask id="mask">
    <g transform="translate(100,100)">
        <rect width="150" height="150" fill="white" />
    </g>
    </mask>
</defs>

<rect x="100" y="100" width="100" height="100" fill="red"/>
<rect x="100" y="100" width="100" height="100" fill="green" mask="url(#mask)" />

</svg>