IE6 lost that battle because it stagnated - it wasn't just not keeping up with new standards, it wasn't innovating in general. We don't know if it turned out the way it did, if Microsoft kept proactively extending HTML in new IE versions. But given that this is largely what was happening in the IE/NN mortal combat era that preceded IE dominance, I'd expect "this browser best works in ..." to make a comeback. And, indeed, it's increasingly common to see websites explicitly say that they want Chrome. Especially new Google projects...