According to Ars Technica, “the game makes heavy use of “HTML5 Canvas elements to render a tile-based 2D world, HTML5 audio APIs to support sound effects, Web Sockets to facilitiate communication with the backend server, and localStorage to save the player’s progress.” The game’s been pretty fun, albiet a little slow to load and connect to a server. If you’re going to try out BrowserQuest, expect the connection times to be a bit long. I almost gave up before it even launched.
On another note, can you believe that you can play an entire video game directly in your browser without Flash? Had someone told me five or six years ago that HTML could completely replace Flash, I would have laughed in their face.