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<!doctype html>
<html>
  <head>
    <script>
      function ns_resolver(prefix) {
        var ns = {
          'uuk': 'http://uukgoblin.com/namespace'
        };
        return ns[prefix] || null;
      }
      function search() {
        var res = document.evaluate('//p[@uuk:attr="val"]', document, ns_resolver, XPathResult.ANY_TYPE, null);
        var first = res.iterateNext();
        if(first === null) {
          alert("not found");
        }
      }
    </script>
  </head>
  <body>
    <div xmlns:uuk="http://uukgoblin.com/namespace">
      <p uuk:attr="val">hello</p>
    </div>
    <input type="button" value="clickme" onclick="search()" />
  </body>
</html>