var Mac=(navigator.userAgent.indexOf("Mac")!=-1);
var Win=(navigator.userAgent.indexOf("Win")!=-1);
var Firefox=(navigator.userAgent.indexOf("Firefox")!=-1);
var Safari=(navigator.userAgent.indexOf("Safari")!=-1);
var Netscape=(navigator.appName.indexOf("Netscape")!=-1);
var Explorer=(navigator.appName.indexOf("Explorer")!=-1);
var Version=navigator.appVersion.charAt(0);

if
(Mac && Firefox) //Reading of Fire Fox
{document.write('<link rel="stylesheet" href="../css/basic_ff.css" type="text/css">');}
else if
(Mac && Netscape && !Safari && Version>="4") //Reading of Netscape
{document.write('<link rel="stylesheet" href="../css/basic_nn.css" type="text/css">');}
else if
(Win && Explorer) //Reading of WinIE
{document.write('<link rel="stylesheet" href="../css/basic_winie.css" type="text/css">');}