NamedNodeMap.removeNamedItemNS()

removeNamedItem(namespace, localName);

const parser = new DOMParser();
const xmlString = '<warning ob:one="test" xmlns:ob="http://www.example.com/ob">Beware!</warning>';
const doc = parser.parseFromString(xmlString, "application/xml");

const pre = document.getElementsByTagName("pre")[0];
const warning = doc.getElementsByTagName("warning")[0];
const attrMap = warning.attributes;

let result = "The `ob:one` attribute initially contains `" + attrMap["ob:one"].value + "`.\n";

result += "We remove it.\n\n";
attrMap.removeNamedItemNS("http://www.example.com/ob", "one");

if (attrMap["ob:one"]) {
  result += "And `ob:one` still exists.";
}
else {
  result += "And `ob:one` is no more to be found.";
}

pre.textContent = result;