url = 'http://cdn.sstatic.net/stackoverflow/all.css?v=cd72db0f0255';
xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
    if (xhr.readyState == 4) {
        alert('xhr.responseText.length = ' + xhr.responseText.length);
    }
};
xhr.open('GET', url, true);
xhr.send(null);