Sure! Here's an example of a simple 2D platformer game implemented as a single HTML file using JavaScript. You can copy the code into an HTML file and run it in a web browser:
This code will create a canvas element where you can control a blue square (representing the player) using the arrow keys. The player can jump on the green platforms. If the player falls off the screen, a "Game Over" message will appear, and you can start the game again by clicking OK in the alert box.
Note: This is a basic example to get you started with a 2D platformer game. You can enhance and extend the game logic and visuals as per your requirements.