Sure, here's an example of a simple Flappy Bird game implemented as a single HTML file using JavaScript. Please note that due to limitations in this text-based format, you won't be able to interactively play the game here, but you can copy the code and save it as an HTML file on your device to play it.
To play the game, copy the entire code and save it as an HTML file on your device. Then, open the HTML file in a web browser (preferably on a smartphone) to play Flappy Bird. Press the spacebar or tap the screen to make the bird jump and avoid the pipes. The score is displayed on the top-left corner, and the game will end if the bird collides with the pipes or goes out of bounds.