var Netscape, MSIE, Opera, Unknown;
  var Win, Mac, Other;
  var NetscapeVer, MSIEVer, OperaVer;
  var OS, Nav, Ver;

// detect browser
  Netscape = navigator.appName == "Netscape";
  MSIE = navigator.appName == "Microsoft Internet Explorer";
  Opera = navigator.userAgent.indexOf("Opera") > -1;
  Unknown = !(Netscape || MSIE || Opera);
  
// detect platform
  Win = navigator.userAgent.indexOf("Win") > -1;
  Mac = navigator.userAgent.indexOf("Mac") > -1;
  Other = !(Win || Mac);
  
// now extract version numbers
  if(Netscape) {
    NetscapeVer = parseFloat(navigator.appVersion);
  }
  else if(MSIE) {
    n = navigator.userAgent;
    MSIEVer = n.substr(n.indexOf("MSIE ")+("MSIE ").length, 4);
    MSIEVer = parseFloat(MSIEVer); 

// converts it into a floatint point number
    if(Opera) {
      OperaVer = n.substr(n.indexOf("Opera ")+("Opera ").length, 4);
      OperaVer = parseFloat(OperaVer);
    }
  }

// now do something!
  if(Win) {
	OS="win";
  }
  else if(Mac) {
	OS="mac";
  }
  
  if(Netscape) {
	Nav="ns";
	Ver=NetscapeVer;
// detects Netscape Version    
    if(NetscapeVer < 4) {
  	Nav="ns";
	Ver=NetscapeVer;
    }
  }
  else if(MSIE) {
	Nav="ie";
	Ver=MSIEVer;
    
  }

// final test 
 if ((OS=="mac" && Nav=="ie")||(Nav=="ns" && Ver<5)){
 	document.location.href="erreur.html" 
	}