Edit in JSFiddle

var loadCSS = function(url, callback){
    var link = document.createElement('link');
        link.type = 'text/css';
        link.rel = 'stylesheet';
        link.href = url;

    document.getElementsByTagName('head')[0].appendChild(link);

    var img = document.createElement('img');
        img.onerror = function(){
            (callback) ? callback(link) : alert('style sheet loaded!');
        }
        img.src = url;
}

var input = document.getElementById('cssinput');

input.addEventListener('keypress', function(e){
   if(e.keyCode && e.keyCode == 13) loadCSS(input.value);
}, false);

<span>Type in a CSS file's location and hit enter to load it:</span>
<input id="cssinput" type="text" style="width: 100%;" value="http://www.backalleycoder.com/big-css-file.css" />