I don't find it as silly as you do. After all, every programming language needs some physical stuff in the real world to follow the directions the language is encoding. For CSS, there's not a standard runtime, because it's obviously not the intention of CSS. But requiring a human to dumbly follow the rules encoded in this purpose-built CSS is pretty reasonable in this context (which is showing things to be Turing complete which one wouldn't expect to be Turing complete).