As an a proof of concept and an entertaining pasttime, 6 friends decided do implement the classic game from 1985 Super Mario Bros. in HTML 5. They not only recreated the whole new game with all the levels, they made it in widescreen, with a random level generator and also a tool for creation of your own levels.
All you need to play is an internet browser that supports HTML 5. They recommend Google Chrome.
Nintendo is not happy with the project, so it is possible that the website will be down pretty soon. In any case, it is open source and you can download the full thing at their GitHub page and who knows maybe learn a little about the wonders of HTML 5.
On the other hand, if you are just interest in playing, all you have to do is click and visit www.fullscreenmario.com