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chromium/third_party/blink/web_tests/fast/frames/avoid-forced-layout-on-empty-subframe.html 

<!DOCTYPE html>
<script src="../../resources/js-test.js"></script>
<body>
<script>
  description("Test that inserting an empty iframe doesn't force a style-recalc/layout");

  var jsTestIsAsync = true;
  window.addEventListener("load", () => {
    window.iframe = document.createElement("iframe");
    window.div = document.createElement("div");
    document.body.appendChild(div);
    shouldBe("internals.nodeNeedsStyleRecalc(div)", "true");
    window.layoutCountBefore = internals.layoutCountForTesting();
    document.body.appendChild(iframe);
    shouldBe("internals.layoutCountForTesting()", "layoutCountBefore");
    shouldBe("internals.nodeNeedsStyleRecalc(div)", "true");
    shouldBe("internals.nodeNeedsStyleRecalc(iframe)", "true");
    finishJSTest();
  });
</script>