Sys.Browser.documentMode Field
Gets a value that indicates the document compatibility mode of the browser.
var mode = Sys.Browser.documentMode;
Return Value
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)
Remarks
The document compatibility modes of Internet Explorer enable different features and can affect the way that content is displayed.