https://github.com/elitheeli/oddities/blob/master/rule110-grid.html

For those confused, here's a proper explanation. No real-world thing can actually be Turing complete (able to express basically any computation that we might want to perform of any size). That's because there are finitely many atoms in the universe, so we can only construct machines of finite size.

It's well known that Rule 110 (Google it) is Turing Compete.

What I've done is made an implementation of Rule 110 in HTML and CSS. Since CSS can't actually really manipulate state, some user interaction is required to "drive" it. In the one that bgruber linked to, it's clicking.