chromium/third_party/blink/web_tests/svg/custom/mask-inside-defs.svg

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <mask id="mymask">
    <rect width="100" height="100" fill='white' />
  </mask>
  <g mask="url(#mymask)">
    <rect x="0" y="0" width="200" height="200" fill="red"/>
    <rect x="0" y="0" width="100" height="100" fill="green"/>
  </g>
</svg>