Kevin Kuchta created a two-way web chat using only CSS and HTML without using JavaScript. The developer from San Francisco created the CSS-only chat on his GitHub page. The chat works by combining background-images loaded via pseudoselectors and a forever-loading index page which keeps the discussion going and the messages going between browsers. It is not a finished product but, with some extra polishing and a better way to type text, it could actually be a feasible idea to use as a web-based support chat.
Source: https://www.bleepingcomputer.com/news/software/developer-creates-a-two-way-chat-using-only-css-and-html/