HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>replace namespaced Attr</title>
  <style>body{font-family:sans-serif;margin:1rem;}</style>
</head>
<body>
  <div id="item" xmlns:special="http://www.mozilla.org/ns/specialspace" special:specialAlign="old">Item</div>
  <pre id="out"></pre>
  <script>
    var MY_NS = "http://www.mozilla.org/ns/specialspace";
    var el = document.getElementById("item");
    var next = document.createAttributeNS(MY_NS, "special:specialAlign");
    next.value = "new";
    var replaced = el.setAttributeNodeNS(next);
    document.getElementById("out").textContent = replaced.value + "\n" + el.getAttributeNS(MY_NS, "specialAlign");
  </script>
</body>
</html>

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Element.setAttributeNodeNS — replace Attr (Try It)