function popup(page,ht,lg){
	x=(screen.width-lg)/2;
	y=(screen.height-ht)/2;
	var popup = window.open(page,"pop","width="+lg+",height="+ht+",left="+x+",top="+y+",toolbar=0,scrollbars=yes");
}

<!-- Cas si l'ordinateur est un PC -->

if (navigator.platform.indexOf("Win")  != -1) {
		if ((navigator.appName=="Netscape") && (parseInt(navigator.appVersion)==4)){
			{
				document.write("<link rel='stylesheet' href='styles/styles_ns.css'>");
			}
		}
		else{
			document.write("<link rel='stylesheet' href='styles/styles_ie.css'><link rel='stylesheet' href='../styles/styles_ie.css'>");
			}
		}
<!-- Cas d'un autre ordinateur -->
else if (navigator.platform.indexOf("Win") == -1) {
	document.write("<link rel='stylesheet' href='styles/stylesmac.css'>");
}

var isN6 = ((navigator.appName == "Netscape") && (navigator.userAgent.indexOf("Mozilla") != -1) && (parseFloat(navigator.appVersion) >= 5) && (navigator.javaEnabled())) ? true : false;