css/css-transitions/dynamic-root-element.html

<!doctype html>
<meta charset="utf-8">
<link rel="help" href="https://drafts.csswg.org/css-transitions/#starting">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1816672">
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<iframe></iframe>
<script>
  let t = async_test("Transitions don't incorrectly start on dynamic document element insertion");
  window.onload = t.step_func_done(function() {
    let frame = document.querySelector("iframe");
    let doc = frame.contentDocument;
    doc.open();
    doc.write("<!doctype html>");
    let root = doc.createElement("html");
    root.innerHTML = `
      <style>
        div {
          width: 100px;
          height: 100px;
          background-color: blue;
          transition: margin-left 10s ease;
          margin-left: 100px;
        }
      </style>
      <div></div>
    `;
    doc.appendChild(root);
    doc.close();
    assert_equals(frame.contentWindow.getComputedStyle(doc.querySelector("div")).marginLeft, "100px", "Transition shouldn't have started");
  });
</script>