Atari Arcade implements three classic Atari games (Pong, Missile Command, and Super Breakout), as entirely browser based games. Just visit atari.com/arcade from any Windows Phone 8 device and you can try it out for yourself.
Do note that you'll have to make sure that Internet Explorer is set to mobile mode, rather than desktop, in the app's settings. You'll also need to make sure your device is rotated into landscape mode. Once the page and games have loaded you'll find an experience that, address bar aside, makes it easy to forget you're running the game in a browser.
While it's a cool technology demo there's also an important message - Windows Phone now has a browser that can offer a first class web app experience. What stands out most is the improved touch input support, without which these games wouldn't be possible (or at least not in such a smooth way).
To whet your appetite here a screenshot from each of the games: