var browser = navigator.userAgent.toLowerCase();
<!-- netscape 4 -->
if (document.layers)
   document.write("<LINK REL='stylesheet' HREF='../risorse/Style_moz.css' TYPE='text/css'>")
      else
<!-- Internet Explore -->
         if (document.all)
            document.write("<LINK REL='stylesheet' HREF='../risorse/Style.css'  TYPE='text/css'>")
         else
<!-- Mozilla -->
            if (!document.all && document.getElementById)
               document.write("<LINK REL='stylesheet' HREF='../risorse/Style_moz.css'  TYPE='text/css'>")
            else
               document.write("<LINK REL='stylesheet' HREF='../risorse/Style_opera.css'  TYPE='text/css'>")