var agent = navigator.userAgent ;

var userOs	= null ;

if ( agent.indexOf('Win') != -1 ){
userOs = "Win";
}
else if ( agent.indexOf('Mac') != -1 ){
userOs = "Mac" ;
}
else{
userOs = "other" ;
}

var macUrl, winUrl, cssSrc ;
	
macUrl = "http://denjoy.ms.u-tokyo.ac.jp/~tudateru/common/mac_en.css";
winUrl = "http://denjoy.ms.u-tokyo.ac.jp/~tudateru/common/win_en.css";

if ( userOs == "Mac" ) {
cssSrc = '<link rel="stylesheet" type="text/css" href="' + macUrl + '">' ;
}
else{
cssSrc = '<link rel="stylesheet" type="text/css" href="' + winUrl + '">' ;
};

document.write( cssSrc );