What url appears below?
<pre></pre>
<iframe src="subdir/subframe.html"></iframe>
<script>
  onload = function() {
    var d = document.createElement("div");
    d.setAttribute("style", "background: url(abc)");
    var d2 = frames[0].document.importNode(d);
    document.querySelector("pre").textContent = frames[0].getComputedStyle(d2).backgroundImage;
  }
</script>