HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createAttributeNS xml:lang</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <p id="greeting">Bonjour!</p>
  <button id="btn">Add xml:lang="fr"</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var el = document.getElementById("greeting");
      var attr = document.createAttributeNS(
        "http://www.w3.org/XML/1998/namespace",
        "xml:lang"
      );
      attr.value = "fr";
      el.setAttributeNode(attr);
      document.getElementById("out").textContent = el.getAttribute("xml:lang");
    });
  </script>
</body>
</html>

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Document.createAttributeNS — xml:lang (Try It)