Browser Quest is a project of the Mozilla developers is aimed at demonstration of capabilities of HTML5 to create browser games running without additional plugins.
BrowserQuest game window
The game is a multiplayer online role-playing game (MMORPG) with a very simple pixel graphics. You play a man armed with a sword, can move around the map in all directions to communicate with other players, sword slashing monsters monsters, collect various bonuses.
The game works in almost all modern browsers, at least in normal browsers Mozilla Fiefox and Chrome it works great! Also the game needs to work on smartphones and tablets.
To play just open in the browser: http://browserquest.mozilla.org/
Details of the project on opennet.ru