Request.Browser.Version doesn't always return the 'correct' browser version for IE because of many factors including compatibility view, meta tags etc.
You would be better off detecting browser capabilities instead. Further reading here: http://msdn.microsoft.com/library/3yekbd5b
As a personal preference, it would be best to detect the OS instead of the browser. Most of the common HTML5/CSS3 features are supported in IE9 and above and IE9 requires Windows Vista and up. The best way to do this using the User Agent is by checking the Trident. If there is compatibility view enabled via the browser or by using meta tags by the app itself, the wrong 'version' is returned but the Trident remains correct.
PS. This isn't fool proof but this would work most of the time.
Further reading about Trident here: http://msdn.microsoft.com/en-us/library/ms537503(v=vs.85).aspx#PltToken