Poorly worded title. Think flash games embedded in webpages.
Just load the game and start playing. Great to kill time.

Has anyone ever tried to do something like this?
It does not seem like a practical thing, but I saw a java GBA emulator and was intrigued.

I'm particularly interested in what kind of performance issues would be encountered.

EDIT: lol so they even have gameboy for phones. http://gameboy.sharejar.com/