Sure, below is an example of a simple open world 2D shooter game implemented as a single HTML file. The game uses JavaScript and is enclosed in script tags:
In this example, the game logic is not included, but you can implement it based on your desired game mechanics and interactions. The game loop is responsible for updating the game state, rendering the game objects on the canvas, and continuously calling itself using `requestAnimationFrame()` to create a smooth animation effect.
You can add additional JavaScript code inside the `gameLoop()` function to handle player input, character movement, enemy AI, collision detection, and other game-specific functionality to create the open world 2D shooter game you envision.
Please note that this code provides a basic template for starting the development of your game. You will need to add more code to create a complete and playable game.