What the fake ipad update screen is
The iPad version differs from the iPhone one in wording rather than design. Updating over a cable brings up a warning not to disconnect the device, which is a genuinely alarming line to see on a tablet somebody was about to use.
A large tablet screen makes this one land well. There is a lot of black with very little on it, and at iPad size that reads as an interruption rather than a picture.
iPad software update prank
Add the page to the iPad home screen and open it from there. It launches without any browser interface, which on a tablet is essentially indistinguishable from the real thing.
Nothing is downloaded and nothing is changed. Closing the page is all there is to undo.
How to run it, and how to stop it
Press F for fullscreen, then F11 to hide the browser’s own toolbars. That second step is the one that matters: an address bar sitting above the screen gives the whole thing away instantly.
To end it: Escape leaves fullscreen, F11 brings the toolbars back, and closing the tab removes it entirely. Nothing is installed, no file is written and no setting is changed, so there is nothing to undo afterwards.
Worth saying plainly — use this on people who will find it funny, and on machines you are allowed to touch. Show them the way out once the joke has landed.