Sure, here's an example of a simple game implemented as a single HTML file with JavaScript:
Tap on the screen to avoid the mad man!
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In this game, there is a mad man (represented by a pictogram) that moves randomly within the game container. The player's goal is to tap on the mad man as many times as possible within a 10-second time limit. The current score is displayed on the screen, and a final score is shown once the game ends. To start the game, the player simply needs to tap on the screen.