CSS has expressions, but not imperative logic or mutability (it's possible I misused the term "control flow")
You're right that HTML (technically, the DOM) has a little bit of intrinsic state, despite our mental model of it being static. However that state is constrained to user interaction and is always very localized, so it avoids most of the maintainability problems that we think of as coming from having mutable state
For example see the css implementation of celluar autonoma http://eli.fox-epste.in/rule110/