var czyWin = (navigator.userAgent.indexOf("Win") != -1);
var czyMac = (navigator.userAgent.indexOf("Mac") != -1);
var czyUnix = (navigator.userAgent.indexOf("X11") != -1);

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<script language="JavaScript" type="text/javascript">
document.write("<style type='text/css'>");
document.write("body {font-size:" + ((czyMac) ? "12" : "10") + "pt}");
document.write("</style>");
</script>
