It's an entirely different concept; this is logic based programming and constraint solving, nothing to do with DOM elements or user input.
Maybe the example of the front page is misleading, but fwir ConstraintJS is a JS library adding constraints to JS, the DOM/CSS are use cases but the library is not limited to those use cases
From [0] and [1], it seems that ConstraintJS does not try to solve constraints, it propagates changes among variables when they change. In particular, variables are not tied to domains.