浏览器判断

var ua = navigator.userAgent.toLowerCase(); 
var isOpera = ua.indexOf("opera") > -1, 
isSafari = (/webkit|khtml/).test(ua), 
isIE = !isOpera && ua.indexOf("msie") > -1, 
isIE7 = !isOpera && ua.indexOf("msie 7") > -1, 
isIE8 = !isOpera && ua.indexOf("msie 8") > -1, 
isGecko = !isSafari && ua.indexOf("gecko") > -1;