opacity-with-mask.html | Explore in Territory

<!DOCTYPE html>
<html>
  <head>
    <style>
      .outer {
        position: relative;
        width: 280px;
        height: 160px;
        background-color: green;
        border-radius:3px;
        overflow:hidden;
        will-change: transform;
      }

      .inner {
        position:absolute;
        top:0;
        left:0;
        right:0;
        bottom:0;
        background-color:red;
        border: 1px solid black;
        opacity: 0.5;
        will-change: transform;
      }
    </style>
  </head>
  <body>
    <div class="outer">
      <div class="inner"></div>
    </div>
  </body>
</html>

chromium/third_party/blink/web_tests/compositing/opacity-with-mask.html