Sure, here's a simple platformer game like Mario, implemented as a single HTML file using JavaScript:
This HTML file creates a simple platformer game where you control a red player box using the arrow keys. The blue blocks represent obstacles. The player's movement is restricted by the blocks, and collision detection is implemented to prevent the player from passing through the blocks.
Note: This is just a basic implementation to give you an idea of a simple platformer game. You can enhance and modify it further based on your requirements and game design.