xSSI and Browser Detection Question

Hello,

I am considering setting up a site using Apache's extended Server 
Side Include functions to serve different versions of the navigation 
structure based upon the user's browser.  I am wondering:

*  If anyone has put together a well tested browser detection test 
using xSSI commands which they would be willing to share.  (For now I 
would be happy simply dividing users into two groups 4+ browsers and 
pre-4 browsers, though ideally I will want to divide them up a little 
more specifically.)

*  If anyone has good advice for why to avoid going down this path 
(or words of wisdom for inevitable problems if I do).

*  If anyone knows how most web caches will handle this and any 
advice for dealing with the resulting problems.

Of course any other suggestions, comments, or useful related 
experiences would also be greatly appreciated.

Thanks,

--Jamey

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