if (navigator.platform.indexOf('Mac') > -1) {
  //mac users
  document.write("<link href='/css/style_mac.css' rel='styleSheet' type='text/css'>");
} else {
  //windows or unix
  if (navigator.appName.indexOf('Microsoft') > -1) {
    //explorer
    document.write("<link href='/css/style_ie.css' rel='styleSheet' type='text/css'>");
  } else {
    //netscape
    document.write("<link href='/css/style_ns.css' rel='styleSheet' type='text/css'>");
  }
}