Sure, here's an example of a simple multiplayer FPS game implemented as a single HTML file using JavaScript:
This implementation sets up a canvas and uses a game loop to continuously draw the player and enemies on the screen. The player can move using arrow keys, and if the player collides with an enemy, the game ends.
Note: This is a basic example to demonstrate the game mechanics. To turn it into a full-fledged multiplayer FPS game, you would need server-side logic to handle multiple players, networking, and more advanced game mechanics such as shooting, scoring, and synchronization.