Sure! Here's an example of a reaction time game implemented as a single HTML file using JavaScript:
Reaction Time Game
In this game, a red square represents the target. When the game starts, the square changes its color to blue, and the player's goal is to tap/click on the square as quickly as possible. The game measures the reaction time (in milliseconds) and displays it in an alert message. After that, a new target is generated after a random interval between 1 and 3 seconds.
You can save the above code in an HTML file and open it in any web browser, including on smartphones, to play the game.