doc = (new DOMParser).parseFromString('<r xmlns="uri1" xmlns:xx="uri2"><b/></r>', 'text/xml');
doc.querySelector('b').setAttributeNS('uri2', 'foo', 'bar');
(new XMLSerializer).serializeToString(doc);
// "<r xmlns="uri1" xmlns:xx="uri2"><b xmlns:ns14641696="uri2" ns14641696:foo="bar"/></r>"

doc = (new DOMParser).parseFromString('<r xmlns="uri1" xmlns:xx="uri2"><b/></r>', 'text/xml');
doc.querySelector('b').setAttributeNS('uri2', 'xx:foo', 'bar');
(new XMLSerializer).serializeToString(doc);
// "<r xmlns="uri1" xmlns:xx="uri2"><b xx:foo="bar"/></r>"

doc.querySelector('b').setAttributeNS('uri2', 'foo2', 'bar2');
(new XMLSerializer).serializeToString(doc);
// "<r xmlns="uri1" xmlns:xx="uri2"><b xx:foo="bar" xmlns:ns14641696="uri2" ns14641696:foo2="bar2"/></r>"