What the fake windows 8 update screen is
Windows 8 introduced the full-screen blue update with a dot spinner, and Windows 10 kept it almost unchanged. The wording is the giveaway between the two: Windows 8 says "Configuring Windows updates", Windows 10 says "Working on updates".
It is a less-searched screen than its neighbours, which makes it a better prank on anyone who would spot a Windows 10 fake immediately.
Windows 8 update prank in a browser tab
Press F11 to hide the browser toolbars before you walk away. That single step is the difference between a convincing prank and an obvious one.
Nothing here touches the machine. It is a page, and closing the tab ends it completely.
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.