Okay, here's a simple April Fool's prank: a fake "System Update" screen that never finishes.

When you run this, it will show a blue screen resembling a system update, with a progress bar that slowly fills up but never quite reaches 100%. The status messages will change randomly. It also tries to disable the cursor and some escape keys for added effect (though browser security might limit this).

Okay, here's another one: a fake "Virus Detected" alert.

This creates a page with some placeholder text. After a couple of seconds, a fake "Virus Detected" warning pops up, complete with a progress bar and silly virus names. It includes a close button that works and reveals the "April Fools!" message after a short delay. There's also a "DO NOT CLICK!" button for extra temptation, which triggers a simple alert confirming the prank.