<!DOCTYPE html>
<style>
  #subframe {
    width: 400px;
    height: 500px;
    background-color: blue;
    position: relative;
  }

  #scroller {
    height: 200px;
    width: 300px;
    position: absolute;
    overflow: scroll;
  }

  #clip {
    position: absolute;
    clip: rect(0px, 100px, 100px, 0px);
  }
</style>
<script>
  onload = function() {
    // Render one frame with clipping, then remove the clip.
    window.requestAnimationFrame(function() {
      window.requestAnimationFrame(function() {
        document.getElementById("clip").style.clip = "auto";
        if (window.testRunner)
          testRunner.notifyDone();
      });
    });
  }

  if (window.testRunner && window.internals) {
    testRunner.waitUntilDone();
    internals.settings.setPreferCompositingToLCDTextEnabled(true);
  }

</script>

<div id="scroller">
  <div id="clip">
    <iframe id="subframe"></iframe>
  </div>
</div>