Sure! Here's a basic example of a football game implemented as a single HTML file using JavaScript:
This is a simple game where you control a player represented by a rectangle at the bottom of the canvas, and you need to hit the ball (represented by a circle) back and forth, preventing it from going off the canvas. The player can be controlled using the left and right arrow keys on a smartphone.
Please note that this is a basic example, and you can enhance it further by adding scoring, levels, and more gameplay features as per your requirements.