if (navigator.appVersion.indexOf("MSIE") > -1 &&  getInternetExplorerVersion() < 8) {
    document.write('<link rel="stylesheet" type="text/css" href="stile.css" media="screen" />');
}
else  {
    document.write('<link rel="stylesheet" type="text/css" href="stile.css" media="screen" />');
    document.write('<link rel="stylesheet" type="text/css" href="stile_ff.css" media="screen" />');
}

function getInternetExplorerVersion()
// Returns the version of Internet Explorer or a -1
// (indicating the use of another browser).
{
  var rv = -1; // Return value assumes failure.
  if (navigator.appName == 'Microsoft Internet Explorer')
  {
    var ua = navigator.userAgent;
    var re  = new RegExp("MSIE ([0-9]{1,}[\.0-9]{0,})");
    if (re.exec(ua) != null)
      rv = parseFloat( RegExp.$1 );
  }
  return rv;
}