Sure, here's an example of a simple game implemented in a single HTML file using JavaScript:
In this game, you control a player represented by a blue square. The objective is to reach the flag (represented by a red square) within 30 seconds. You can use the arrow keys on your smartphone to move the player in the corresponding direction (right, left, up, or down). The game ends when you either reach the flag or the time runs out. The remaining time is displayed at the top of the game container.