var Win=(navigator.userAgent.indexOf("Win")!=-1);
var Mac=(navigator.userAgent.indexOf("Mac")!=-1);

if
(Win)
{document.write('<LINK rel="stylesheet" href="/css/font_win.css" type="text/css">');}

else if
(Mac)
{document.write('<LINK rel="stylesheet" href="/css/font_mac.css" type="text/css">');}