Here's an example of a simple game in a single HTML file that meets your requirements. The player controls a sword in the middle of the screen and must kill 25 randomly spawning demons to win. The game is designed for smartphone operation and uses pictograms for visuals.
To play the game, simply save the code above as an HTML file and open it in a web browser, preferably on a smartphone. The game will start, and you can move the sword by dragging your finger on the screen. Tap on demons to kill them. Once you have killed 25 demons, an alert will be displayed notifying you that you have won the game.