Sure, here's an example of a simple game implemented as a single HTML file, using JavaScript and enclosed in script tags:
In this game, you simply have to tap/click on the big blue area to increase your score. The score is initially set to zero, and every time you tap/click, it increases by one. The current score is displayed in the middle of the screen using a large font size.
Please note that this is a very basic implementation, and you can modify it further to add more complex game mechanics, graphics, and features based on your requirements.