var
  userAgent = navigator.userAgent,
  isKHTML = (userAgent.indexOf('KHTML') >= 0),
  isOpera = (userAgent.indexOf('Opera') >= 0),
  isIE = (!isOpera && userAgent.indexOf('MSIE') >= 0),
  isIE50 = (isIE && /MSIE 5\.0/.test(userAgent) && navigator.platform == 'Win32'),
  isMozilla = (!isKHTML && !isOpera &&  userAgent.indexOf('Gecko') >= 0);