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?

Also, from the page ppk links to:

   <link rel="transition-enter" duration="0.25s" href="URI" />

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?