[d4tocchini]

GSS author here. What we're working on is a lot crazier than that! Using phantom.js we precompute layout for every possible screen size & generate pure CSS.

Precomputation - From small phones to large desktop, there are ~2million width & height combinations, even if solving a single screen size takes 10ms, were talking ~6hours. To get around this, using a sort of bisection method, we selectively solve for as few screen sizes as possible to test linearity of each variable's solved values. With Cassowary each variable is linear between breakpoints, making this possible.

Pure CSS Generation - Generate media queries & styles with calc() where calc is a linear function with respect to vw & vh units. A little JS we only be needed for browsers that don't support calc or viewport units.

Limitations - Precomputation will only work for deterministic / static layouts. For a highly volatile DOM, runtime will be needed, but that's probably OK given the fact that you're already using JS to mutate things in runtime.