<!DOCTYPE html>
<html>
<head>
<style>
  .box {
    width: 150px;
    height: 150px;
  }
  #ref {
    background-color: rgb(153,255,153);
  }
</style>
</head>

<body>
<!-- You should see two squares of the same color -->

<embed class="box" type="application/x-webkit-test-webplugin" background-color="green" opacity="0.4">
<div id="ref" class="box"></div>
</body>
</html>