Sure, here's an example of a simplified version of the game "Flappy Bird" implemented in a single HTML file using JavaScript:
You can save this code to a file with a ".html" extension and open it in a web browser to play the game. The game can be played by pressing the spacebar (keyboard) or tapping the screen (touchscreen devices) to make the bird jump and avoid colliding with the pipes. The score increases each time the bird passes through a set of pipes. If the bird collides with a pipe or hits the ground, the game will be over.