There is a default namespace declaration xmlns="http://www.w3schools.com"
in the input that you need to take into account with e.g.
var nodes = xmlDoc.evaluate("df:blog/df:Title", xmlDoc, function(prefix) { if (prefix === "df") return "http://www.w3schools.com"; }, 5, null);
var result;
while ((result = nodes.iterateNext()) != null)
{
document.write(result.textContent);
}