HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createAttribute lowercase</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">Create My_Custom_Attr</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var attr = document.createAttribute("My_Custom_Attr");
      document.getElementById("out").textContent = [
        "name: " + attr.name,
        "localName: " + attr.localName
      ].join("\n");
    });
  </script>
</body>
</html>

Lines: 0 | Characters: 0

Live Preview Auto-refresh on Run

Document.createAttribute — lowercase name (Try It)