What the fake iphone update screen is
Apple removed the percentage from the iPhone update screen years ago, leaving a bar with no numbers and no estimate. That absence is the detail people remember, and putting a percentage on a fake iOS screen gives it away instantly.
On a phone, add this page to your home screen first. It then opens without any browser interface at all, which is by some distance the most convincing way to run it.
iOS update prank on an iPhone
iPhones do not expose the fullscreen API the way desktop browsers do, so the page expands to fill the screen instead. It is close, but the status bar stays visible unless the page has been added to the home screen.
Nothing is installed and no permission is requested. Swiping away the page ends it.
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.