Looks appealing, but it is very confusing. At least with Sass/less etc you can easily distinguish between native css and non-native css constructs such as mixins and extends. But the additions rucksack make are ambiguous. Is it real spec, future spec or something that rucksack developers have added for convenience?<p>I'd prefer nextCSS with all the spec being based on what is actually going to make in into CSS. Otherwise it's the same downsides as preprocessing languages with the added bonus of not being able to easily tell what lines are native to css or not.