HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createTreeWalker elements</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <div id="demo">
    <ul>
      <li>One</li>
      <li><strong>Two</strong></li>
    </ul>
  </div>
  <button id="btn" type="button">List element tags</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var root = document.querySelector("#demo");
      var walker = document.createTreeWalker(root, NodeFilter.SHOW_ELEMENT);
      var tags = [];
      while (walker.nextNode()) {
        tags.push(walker.currentNode.tagName.toLowerCase());
      }
      document.getElementById("out").textContent = tags.join(", ");
    });
  </script>
</body>
</html>

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Document.createTreeWalker — list elements (Try It)