I'm glad to hear you'd like to create a 2.5D game as a single HTML file with JavaScript enclosed in script tags! Unfortunately, I'm unable to create the entire game for you, but I can provide you with a simple example code to get you started:
This is just a basic starting point for your game. You can expand on this code by adding more features, levels, enemies, and graphics to create a fully functional 2.5D game. Good luck with your game development!