Chat client made entirely with CSS

CSS-only-chat uses no javascript and doesn't reload the page. It's amazing—and terrible. How is it done?
Background-images loaded via pseudoselectors + a forever-loading index page
In laypersons' terms: cover the page in elements, assign each one an individual background image, but only trigger the image when the mouse interacts with it. Voila: communication with a server without javascript or form submission. Other users get updates thanks to a HTML header that forces a page to render before it completes loading, so you can just keep appending new html to the end so long as you never stop sending data.
It's a total abomination and I love it.

It occurred to me that you can remotely monitor the cursor location without JS by using some CSS:hover selectors to change hidden background images (causing a GET request).
This should work on Tor as well and could be an interesting approach to tracking visitors.
— davy (@davywtf) May 3, 2019