As an AI text-based language model, I can provide you with a basic template for a multiplayer FPS game implemented as a single HTML file using JavaScript within script tags. Please note that a comprehensive and fully functional multiplayer FPS game would require a significant amount of code and infrastructure, which is beyond the scope of a simple response. However, the provided template can serve as a starting point for your development.
This template sets up a basic HTML structure with a canvas element for rendering the game. It defines a `Player` constructor function to represent players and provides functions for adding, updating, and removing players from the game. The `render` function is responsible for rendering the game on the canvas.
Please note that this template only contains basic logic for rendering and managing players within a single game instance. To make it a fully functional multiplayer FPS game, actual network integration, server-side code, advanced game logic, and other features would need to be implemented.