gecko-dev/gfx/webrender/tests/bug_203a.html

<style>
  body {
    margin: 0;
  }
  iframe {
    position: absolute;
    top: 20px;
    left: 20px;
    width: 200px;
    height: 200px;
  }
</style>
<iframe src="bug_203b.html"></iframe>