Sys.Browser.documentMode Field
.NET Framework 4
Gets a value that indicates the document compatibility mode of the browser.
var mode = Sys.Browser.documentMode;
The value 0 for any browser other than Internet Explorer. For Internet Explorer, returns one of the following numbers that indicates compatibility with a version of Internet Explorer:
5 (compatible with Internet Explorer 5.0)
7 (compatible with Internet Explorer 7.0)
8 (compatible with Internet Explorer 8.0)