var ua = navigator.userAgent //まず、userAgentの情報を取得する

    var cssHTML = '<link rel="stylesheet" type="text/css" href="'

    //Macならmac.css、Winならwin.css、その他ならall.cssを使う
    if( ua.indexOf("Mac") != -1 )      cssHTML += '../styles/mac.css'  
    else if( ua.indexOf("Win") != -1 ) cssHTML += '../styles/master.css' 
    else                               cssHTML += '../styles/master.css' 

    cssHTML += '">'

    document.write(cssHTML)