Sure, here's an example of a simple shooting game implemented as a single HTML file:
This game is optimized for smartphone operation. It allows the player to control the player character (represented by a green circle) by touching and dragging on the screen. Tapping the screen will fire a bullet (represented by a red circle) from the player's position towards the top of the canvas. The bullet will continue moving until it hits the top or goes off-screen.
Please note that this is a very basic implementation and can be expanded upon to add features like enemies, scoring, sound effects, and more.