What the fake windows 98 update screen is
The teal desktop is one of the most recognisable colours in computing, and it does most of the work here. Add a blocky progress bar and a font stack starting with MS Sans Serif and the era is unmistakable.
This is the furthest back the set goes. It is a period piece rather than a serious prank, and it is very good on a display at a retro-themed event.
Windows 98 update prank screen
Nobody is going to believe a modern laptop is running Windows 98, so treat this as a visual joke rather than a deception.
It scales cleanly to a television, which makes it a good period backdrop for a party or a display.
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.