Sure, here's a simple game in a single HTML file using JavaScript:
Number Game
Score: 0
Explanation:
This code creates a simple game where numbers continuously increase every 1 second. The score starts at 0 and is displayed on the screen. Every second, the score increases by 1 and a new number is spawned below the score. The new number is always the same as the score. The score is updated using the `updateScore()` function and a new number is spawned using the `spawnNumber()` function. The game loop is started using `setInterval()` function.
The game is designed for smartphone operation and can be played by opening the HTML file on a smartphone browser. The score is displayed in a heading tag (`<h1>`) and the spawned numbers are shown as separate `<div>` elements with a light blue background.