Sure! Here's an example of a simple Flappy Bird game implemented as a single HTML file using JavaScript. Note that the game mechanics can be adjusted and improved as per your requirements.
You can save this code in a file with an `.html` extension, and open it in a web browser to play the game. Simply tap/click on the screen to make the bird flap and try to avoid the pipes. The score increases every time the bird successfully passes through a pair of pipes.