“Game over”. In this day and age of complex games with storylines and career modes that last for tens of hours, it’s not really a concept that has a lot of relevance. However, in the golden age of the arcade, those two words made it very clear that your time was up and it was time to find another quarter. Home games of this era were similarly blunt, and if you couldn’t rise to the challenge, you’d be seeing the death screen more often than not.
[foone] continues to maintain the site, and adds new games from time to time. Animated GIF support has been a recent addition for screens like Metroid and Bad Dudes, and there are even character choices for Super Mario Bros. The code is available on Github if you feel the need to tinker yourself.