if (document.layers && navigator.platform.match(/Win/)) { // Win N4
    document.write('<link rel="stylesheet" type="text/css" href="css/winn4.css">');
} else if (document.layers && navigator.platform.match(/Mac/)) { // Mac N4
    document.write('<link rel="stylesheet" type="text/css" href="css/macn4.css">');
} else { // Others
    document.write('<link rel="stylesheet" type="text/css" href="css/normal.css">');
}