Javascript Event Normalization

function Event(e) {
    e = e || (e = window.event);
    e.target = e.target || e.srcElement;
    e.targetNN = event.target.nodeName.toLowerCase();
    e.preventDefault = e.preventDefault || function () { e.returnValue=false; };
    e.stopPropagation = e.stopPropagation || function () { e.cancelBubble=true; };
    return e;
}

document.body.addEventListener('mousedown', function (e) {
    event = Event(e);
    if (event.targetNN == 'span') {
        event.preventDefault();
    };
});