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feature detection happens too late, if you want to serve fast.

I could have sworn that some compat modes fairly faithfully mimic the original version- to the point of dropping APIs such as addEventListener. So yes, you can use feature detection but if you get flagged by IE then you essentially need to code for IE7 or whatever.

The feature detection mantra is true for you and me, but it's a leap to say you can apply the same tactic to a huge site like Google. Google isn't stupid, if they UA sniff, you can bet they have good reasons.

bs. most fixes for ie are implemented with the conditional comment, maybe even one for each version of ie. loading a truckload of css.

and honestly, for most cases feature detection is just an empty term. i agree with if t for javascript. but the rest is mostly impossible (or so much more work that it is i practical)