Assuming you’re using Chrome, the only correct browser that is. That’s right, Bastion has made the jump to the web already and is playable in your Chrome browser, right now! Just head over to the Chrome store and install it (which only takes a minute or two) then check out the demo if you want to see how it runs. From there you can purchase it if you like.
I already own the game on XBLA and was tempted to buy it during the recent Steam sale, but I have to admit being able to play it in my browser whenever I feel like it is pretty tempting.
I played the demo for a moment and there were no problems running it, which is pretty surprising.