I've just committed a prototype using a "dynamic probes" technique
that available both on the client and server sides.
This is inspired by http://detector.dmolsen.com/ and
https://github.com/yiibu/profile - for now my features detection code
is extremely basic, but we can later merge with what was used in the
previous logcapture prototype (using modernizr and platform.js),
features detection code, which is the hard part IMO.
The rest is just fairly simple plumbing, see  for a description of
the detection scenario (the code is there BTW).
I've uploaded a runnable jar of that prototype , to play with it
start with java -jar that.jar and open http://localhost:8085.
If the DMAP_p cookie is not present or too old (30 seconds for now,
useful for testing), browsing to any page should first redirect to the
DMAP_probe.html page, which runs the features detection code (outside
of any content pages, so conveniently isolated), sets the cookie and
redirects to the originally requested page.