Sure, here's an example of a simple 2D platformer game with falling bullets implemented in a single HTML file using JavaScript:
This code sets up a game window with a player character (red square) and falling bullets (black circles). The player can be moved left or right using the arrow keys, and bullets are continuously created and dropped from the top of the window. If the player collides with a bullet, a "Game Over!" message is displayed.
You can save this code in a `.html` file and run it in a web browser to play the game. Note that this implementation assumes desktop keyboard inputs, but you can adapt it to smartphone touch controls if needed.