Sadly, IE11 no longer has MSIE in its navigator.userAgent:
Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; OfficeLiveConnector.1.5; OfficeLivePatch.1.3; .NET4.0C; BRI/2; BOIE9;ENUS; rv:11.0) like Gecko
As to why you want to know which browser you're using, it's because every browser has its own set of bugs, and you end up implementing browser and version specific workarounds, or tell the user to use a different browser!
