Grid Style Sheets – CSS polyfills from the future

If this is what I think it is (a new layout engine that runs in JavaScript, so that CSS is used just for styling, not layout), it is what I've been waiting YEARS for. Unfortunately, the site is deafeningly silent on specifics. What browsers does it support? Is there any kind of tutorial for getting started? When would this be appropriate to use, and when not? What is performance like with rendering times, both on desktop and mobile? Window resizing?…

As GUI dev who worked with Apple's AutoLayout extensively I can confidently say constraint-based layout is a trap. It looks like more intuitive way to go on simple examples, but complexity spikes quickly as number of controls goes up. It's more complicated, more resilient to changes (harder to maintain), easy to get wrong, hard to understand and debug, bugs generally look uglier (what would be minor misplacement in CSS often becomes overlapping or clipping). It does not support word wrapping…

Can someone explain to me what this is good for in real deployment scenarios? I'm moderately experience with front and back-end stacks, but have little understanding what this is about.

Very interesting css preprocessor/layout engine combination, kudos 1999. Many people annoyed with css might love this quite a bit. I wonder about the performance implications. The layout engine obviously has to run in javascript and thus it will result in a slower startup time (gss has to parse the logic) and the layout itself will be obviously slower than "native css".