Here's a simple tennis game implemented as a single HTML file using JavaScript:
You can copy the above code and save it as an HTML file. Open the file in a web browser to play the tennis game. The game is controlled by the left and right arrow keys on a smartphone or keyboard. The objective is to bounce the ball off the paddle and prevent it from hitting the bottom wall. If the ball hits the bottom wall, the game will be over and an alert will be shown.