Sure, here's a simple game implemented in a single HTML file using JavaScript:
In this game, a black box is displayed on the screen for 2 seconds. The player needs to click on the box to earn points. After the 2 seconds, the box disappears and reappears after 4 seconds. The player's score is displayed at the top (replacing the previous content of the document body) and increases by one every time the box is clicked. The game continues in this pattern.