if ((navigator.appName == "Netscape") && (parseInt(navigator.appVersion) >= 4)) {
                if (navigator.userAgent.indexOf('Win') != -1)
                   document.write("<link rel='stylesheet' href='http://grupotortuguero.org/css/gt_win_nn.css'>");
                else if (navigator.userAgent.indexOf('Mac') != -1)
                   document.write("<link rel='stylesheet' type='text/css' href='http://grupotortuguero.org/css/gt_mac_nn.css'>");
                }

        if ((navigator.appName == "Microsoft Internet Explorer") && (parseInt(navigator.appVersion) >= 3)) {
                if (navigator.userAgent.indexOf('Win') != -1)
                   document.write("<link rel='stylesheet' type='text/css' href='http://grupotortuguero.org/css/gt_win_ie.css'>");
                else if (navigator.userAgent.indexOf('Mac') != -1)
                   document.write("<link rel='stylesheet' type='text/css' href='http://grupotortuguero.org/css/gt_mac_ie.css'>");
                }