var headID = document.getElementsByTagName("head")[0];         
var cssNode = document.createElement('link');
cssNode.type = 'text/css';
cssNode.rel = 'stylesheet';
cssNode.media = 'screen';
if (/Firefox[\/\s](\d+\.\d+)/.test(navigator.userAgent)){ //test for Firefox
cssNode.href = 'inc/css/firefox.css';
}
else if(/Safari/.test(navigator.userAgent)){
cssNode.href = 'inc/css/safari.css';
}
else if (/Opera[\/\s](\d+\.\d+)/.test(navigator.userAgent)){
cssNode.href = 'inc/css/others.css';
}
else {
cssNode.href = 'inc/css/others.css';
}
headID.appendChild(cssNode);