removeNamedItemNS(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.querySelector("pre");
const warning = doc.querySelector("warning");
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");

result += attrMap["ob:one"]
  ? "And 'ob:one' still exists."
  : "And 'ob:one' is no more to be found.";

pre.textContent = result;