chromium/third_party/blink/web_tests/compositing/images/direct-image-background-color.html

<!DOCTYPE>

<html>
<head>
  <title>Directly composited images with background color</title>
  <style type="text/css" media="screen">
    body {
      outline: 10px solid transparent; /* affects layer sizes */
    }

    img {
      display: block;
      margin: 20px;
      height: 180px;
      width: 260px;
      padding: 10px;
      background-color: rgba(0, 0, 128, 0.5);
    }
    
    .composited {
      will-change: transform;
    }
  </style>
</head>
<body>

  <img src="../resources/alpha-gradient-small.png">
  <img class="composited" src="../resources/alpha-gradient-small.png">

  <p>Top and bottom should look the same.</p>

</body>
</html>