On any page, press in order: ↑ ↑ ↓ ↓ ← → ← → B A. Your keyboard layout doesn’t matter. The holographic cat dashes along the bottom of the screen in a couple of seconds. It doesn’t get in the way: clicks go right through it.
Players once typed this combo in Konami games to get a bonus, and it has been hidden everywhere ever since. You’ll need a keyboard — it won’t work on a phone without one.