Versioning (Windows 7 and Windows Server 2008 R2 Application Quality Cookbook)
One of the most common application compatibility issues for Windows Internet Explorer is User Agent (UA) Strings and Version Vectors. Windows Internet Explorer 8 introduces a new UA String to help applications detect which version that users are running. In addition to simply increasing the MSIE value that is associated with Internet Explorer in the UA String, Internet Explorer 8 adds a Trident/4.0 value to help you detect if Internet Explorer 8 is running in Compatibility View mode. These changes, plus hardcoded version checks, can potentially introduce compatibility issues between browsers.