src/cobalt/layout_tests/testdata/css3-transitions/5-simple-transition-for-pseudoelement.html

<!DOCTYPE html>
<!--
 | Test case which ensures that when a pseudo-element can handle CSS
 | transitions being applied to it properly.
 -->
<html>
<head>
  <style>
    #block {
      width: 100px;
      height: 100px;
      background-color: yellow;
      font-size: 30px;
    }
    #block::after {
      content: "";
      display:block;
      transform: translateX(100px);
      width: 100px;
      height: 100px;
      background-color: #0f0;
      transition: background-color 1s linear;
    }

  </style>
</head>
<body>
  <div id="block"></div>

  <script>
    if (window.testRunner) {
      window.testRunner.waitUntilDone();
    }

    var blockDiv = document.getElementById('block');

    window.addEventListener('load', function() {
      if (window.testRunner) {
        // Do a layout upon the load event so that we setup our source styles
        // that we will be transitioning from.
        window.testRunner.DoNonMeasuredLayout();
      }

      // Modify the rule that targets the pseudo-element, to trigger the
      // transition.
      var sheet = document.styleSheets[0];
      var rules = sheet.cssRules;
      rules[1].style.backgroundColor = '#00f';

      if (window.testRunner) {
        // Run the transition a bit so that it is started but not completed.
        window.testRunner.DoNonMeasuredLayout();
        window.testRunner.AdvanceClockByMs(500);

        // Measure that the transition should now appear half-way finished.
        window.testRunner.notifyDone();
      }
    });
  </script>

</body>
</html>