Sure, here's an example of a simple BMX game implemented as a single HTML file using JavaScript:
In this BMX game, you control a bike that moves vertically on the screen. Pressing the Space key makes the bike jump, avoiding collision with the ground. If the bike hits the ground, a Game Over message is displayed and the game restarts. The bike is represented by a rectangle for simplicity.
To play the game, open the HTML file in a web browser, and press the Space key to jump.