Sure! Here is a simple paintball shooting game implemented in a single HTML file:
To play the game, open a web browser and save the above code into a file with a `.html` extension. Then, open the saved HTML file in the browser. You will see a canvas where you can control a blue player rectangle using the left and right arrow keys on your smartphone's keyboard. Press the spacebar to shoot red paintballs and try to hit the green target rectangles moving downwards. Each successful hit will increase your score. The game ends when the target rectangles pass the bottom of the canvas.
Note: This implementation assumes that the smartphone's browser supports HTML5 Canvas and JavaScript.