Here's a (static, that don't flash) screenshot: https://up.l3m.in/file/1645698692.png

edit: and here's a (very slow, 2s transition) gif of what it renders to: https://up.l3m.in/file/1645698812-slow-strobe.gif

It is a page with a schematic depiction of a floor lamp. "Click anywhere to turn the light on/off" is written below. When you click, the light turns on or off depending on the previous state, and background color changes accordingly between dark and light. The on/off state is the same for all visitors of the page, so, since this is on the front page of HN, it flickers quickly. Hence the strobe warning.

It's a webpage with a simple picture of a lamp and the text "40 people in the room, click anywhere to turn the light on/off". The page background alters between dark gray and white depending on whether the light is off and on. It's essentially a collective online light flickering exercise and it's about as strobe as you imagine that would be.