"GSS is a CSS preprocessor & JS runtime that harnesses Cassowary.js, the JS port of the same constraint solving algorithm Apple uses in Cocoa Autolayout for iOS & OS X. GSS & Cassowary are based on “Constraint Programming” - a paradigm where developers focus on declaring the “what” and relying on a mathematical solver to figure out the “how”... This makes Constraint Programming a natural fit for declarative languages like CSS."