Sure, here's an example of a simple Flappy Bird game implemented as a single HTML file using JavaScript:
This HTML file creates a canvas element where the game is displayed. The bird and pipes are represented by simple yellow circles and green rectangles respectively. The player can control the bird by pressing any key or tapping the screen (assuming smartphone operation). The score is displayed on the canvas. The game loop continuously updates the game elements and checks for collisions. The game ends if the bird collides with any of the pipes.