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chromium/third_party/blink/web_tests/fast/css/webkit-mask-image-painting.html 

<!DOCTYPE html>
<html>
  <head>
    <style>
      div {
        background: skyblue;
        width: 100px;
      }
      div span {
        display: block;
        -webkit-mask-box-image: -webkit-linear-gradient(left,black 48px,transparent 60px);
      }
      div > span > b {
        display: inline-block;
        background: black;
        width: 10px;
        height: 10px;
        margin: 1px;
      }
    </style>
  </head>
  <body>
    <div><span><b></b><b></b></span></div>
    <p>
      The blue box above should contain two black boxes.
    </p>
  </body>
</html>