chromium/third_party/blink/web_tests/external/wpt/css/css-masking/mask-image/support/mask.svg

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200">
  <mask id="mask">
    <rect x="50" y="50" width="100" height="100" fill="white"/>
  </mask>
</svg>