What the fake macos update screen is
Apple’s installer shows almost nothing — a mark, a line of text, a hairline progress bar and an estimate. That restraint is what makes it recognisable, and it is also what makes it easy to draw convincingly.
The estimate is the running joke. "About 23 minutes remaining" has been the same lie on every Mac since roughly forever, and it is what most people picture when they think of this screen.
Mac update prank screen, no download
On a Mac, press Control-Command-F to make the browser fullscreen and the illusion is essentially complete — there is no visible interface left to give it away.
Nothing is installed and no permission is requested. It is a page drawing a picture, and closing the tab is the whole cleanup.
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.