AT least, they did something right, the User Agent is specific, so you can serve pages customised to the Vita. For everything else, forget it. To get the 3 hours battery life, they throttled everything.
The build is an old Webkit one.
No <video>, no <audio> no webworkers, no SVG inpage, no WebGL, no Websocket, no FileReader, no Webstorage, no Geolocation... That was two months ago. An update was released recently, but I have no informations on that one.
Anyway, think about that : Sony, as every other console maker loses money on the hardware, and use it to boost software sales. The more time you are on the web to play free games, the more they are losing money. Apple is the contrary. The more time you pass online on non-AppStore webapp/webgame, the less financial burden on them for running and curating the AppStore...
Sony has incentive to push you out of the web, Apple has his interesting in pushing you out of the AppStore.