Sure! Here's a simple example of a 2D platformer shooter game implemented in a single HTML file using JavaScript:
In this game, you control a blue player character and shoot at green enemies. The objective is to eliminate as many enemies as possible, while avoiding collisions with them. The score increases by 10 for each enemy defeated. The game can be played using the arrow keys to move the player character and the spacebar to shoot bullets.
Note: This is a basic example and can be enhanced further with more features, levels, enemy behavior, and additional mechanics.