Sure! Here's a simple implementation of the Sans fight game using JavaScript and HTML in a single HTML file. In this game, you can control a character and try to avoid Sans' attacks.
Save the above code in a file with the extension ".html" and open it in a web browser. You will see a rectangular game area with a white player block at the bottom and a red enemy block at the top. Use the left and right arrow keys to move the player and avoid the enemy. If the player collides with the enemy, a game over alert will be shown, and the game will reset.