HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>document.moveBefore fallback</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <button id="btn" type="button">Run helper</button>
  <pre id="out">Click the button</pre>
  <script>
    function moveUnderDocument(movedNode, referenceNode) {
      if (typeof document.moveBefore === "function") {
        try {
          document.moveBefore(movedNode, referenceNode);
          return "moved with moveBefore";
        } catch (err) {}
      }
      document.insertBefore(movedNode, referenceNode);
      return "moved with insertBefore";
    }
    document.getElementById("btn").addEventListener("click", function () {
      var c = document.createComment("helper-demo");
      document.body.appendChild(c);
      document.getElementById("out").textContent = moveUnderDocument(c, null);
    });
  </script>
</body>
</html>

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Document.moveBefore — insertBefore fallback (Try It)