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? How does it handle boxes that depend on the size of their content? What about non-visible boxes, when browsers often lie because they haven't actually rendered the text to be able to tell?
If anything ever needed a comprehensive FAQ, it's this.
But this has the potential of freeing us from the horrible disaster for layout that is CSS, and giving us a sane replacement, without waiting for browsers or the W3. If it really is what it promises, and works flawlessly, I wouldn't be suprised to see it become as ubiquitous as jQuery.