if (navigator.userAgent.indexOf('WebKit')>-1) {
  document.write('<link rel=\"stylesheet\" type=\"text/css\" media=\"screen\" href=\"css/default.safari.screen.css\" />');
}
if (navigator.userAgent.indexOf('Firefox/3.')>-1) {
  document.write('<link rel=\"stylesheet\" type=\"text/css\" media=\"screen\" href=\"css/default.ff3.screen.css\" />');
}