To output an XML document as a string, simply call the format() function of the Y.XML class:

YUI().use("datatype-xml", function(Y) {
    var xmlString =
        '<myroot>' +
            '<item type="foo">' +
                '<name>Abc</name>' +
                '<rank>1</rank>' +
            '</item>' +
            '<item type="bar">' +
                '<name>Def</name>' +
                '<rank>2</rank>' +
            '</item>' +
            '<item type="bat">' +
                '<name>Ghhi</name>' +
                '<rank>3</rank>' +
            '</item>' +
        '</myroot>';

    var myXMLDoc = Y.XML.parse(xmlString);

    alert(Y.XML.format(myXMLDoc));
});