Sure, here's an example of a simple endless shooter game implemented in a single HTML file using JavaScript:
This game is designed to be played on a smartphone. The player can control a blue square located at the bottom of the screen by tapping and moving their finger. Tapping on the screen shoots bullets from the player square. The goal is to shoot down yellow enemy squares moving from top to bottom while avoiding collision with them. The speed of enemies and bullets increases every minute, making the game harder as time goes on.
Please note that this is a simplified example and there are many possible improvements and additional features that could be added to make the game more enjoyable.