To showcase the power of HTML 5, Mozilla used it to create a complete MMORPG game that can be played right now in your browser along with thousands of players from around the world.
BrowserQuest is not on level with industry giants such as World of WarCraft or Star Wars: The Old Republic, but it is a decent game to spend slightly less than an hour discovering different locations and fighting new enemies along with thousands of players whom you can interact and chat with.
Since BrowserQuest is written in HTML5/JavaScript, it is available across a lot of different browsers and platforms. The game can be played in Firefox, Chrome and Safari. With WebSockets enabled, it's also playable in Opera. Moreover, it's compatible with iOS devices, as well as tablets and phones running Firefox for Android.
The mobile versions are more experimental than the desktop experience, which has richer features and performance, but it's an early glimpse of what kind of games will be coming to the mobile Web in the future.
Even more, BrowserQuest is open source, meaning that all interested developers can download its source code to learn or to expand it.
BrowserQuest can be played here and its source code can be found here.