I haven't looked into it, but most of these tend to do clever tricks with CSS animations. https://dev.to/janeori/expert-css-the-cpu-hack-4ddj is by the same author, IIRC, and explains some of it.

CSS alone is not Turing-complete (AFAIK), because you don't have anywhere to attach state (the computed style) if you don't have a DOM and loops are generally prohibited. But CSS exists literally only to style a DOM, so I don't consider that all that much of a concession.