This was the first I'd heard of "Navigation Transitions" (perhaps I don't read the right mailing lists), and ppk's right - this seems like an absolutely crazy feature to add.
Two things: 1) it requires an extra stylesheet (from what I can make out, at least); 2) you can already achieve this with JavaScript, if you really want it (which is, itself, debatable), so why add extra cruft into the browser?
This makes absolutely no sense, though perhaps it has a place amongst the wealth of gibberish we've seen stuck between <head> tags over the years - <meta http-equiv="pragma">, anyone?
