jsでUAをスマホ、PC、タブレットで振り分ける

<script type="text/javascript"> 
var ua = navigator.userAgent;
if(ua.indexOf('iPhone') != -1 || ua.indexOf('iPad') != -1 || ua.indexOf('Android') != -1){
//iPhone & iPad 
document.write('<link rel="stylesheet" href="style_ip.css">');
}else{
//PC
document.write('<link rel="stylesheet" href="style_pc.css">');
}
</script>