You need to return XML to an API. Unfortunately IE8 and older is still inconsistent with how modern browsers handle XML document creation. You still don't want to create XML by concatenating strings, but you have jQuery.

xmlSerialzing.js

// Assuming that you have JQuery and Underscore in your global namespace
// you have an array of strings you want to convert to an XML document
function toXML(data) { // data = ['string'(, ...)]
    var s = '', $xml;
    // build an empty structure
    // <items><item/><item/>...</items> 
    // yes, this is string concatenation, 
    // but we control the vocabulary
    _(this.length).times(function() { s = s + '<item/>'; });
    s = '<items>' + s + '</items>';
    // parse it into an XML document
    xml = $.parseXML(s);
    $xml = $(xml); // wrap the document so we can use JQuery methods
    
    // now add a text node to each of the 'item' children
    // because attempting to append nodes to $xml fails in IE8
    $xml.find('item').text(function(i, old) {
        return data[i];
    });
    
    // now call the appropriate serializer
    // FIXME: find the IE9+ serializer
    if (window.ActiveXObject) {
        s = xml.xml;
    }   
    else {
        s = new XMLSerializer().serializeToString(xml);
    }
    
    return s;
}