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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>activeViewTransition null</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var el = document.createElement("div");
    document.body.appendChild(el);
    var out = document.getElementById("out");
    if (!("activeViewTransition" in el)) {
      out.textContent = "activeViewTransition not supported";
    } else {
      out.textContent = JSON.stringify({
        activeViewTransition: el.activeViewTransition,
        note: "null means no element-scoped transition is active"
      }, null, 2);
    }
  </script>
</body>
</html>

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Element.activeViewTransition — idle null (Try It)