Written as a proof of concept, here is a playable demo of a Super Mario game with sound which is written in 14kb of Javascript:

"Super Mario in 14kB Javascript" - http://blog.nihilogic.dk/2008/04/super-mario-in-14kb-javascript.html

Very impressive!

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Updated 2021 : note this post is from 2008 and is now out of date. The original link is now dead, so I have unlinked it.