Can I run this in-browser without the command line? LESS is very easy to get started with - just include less.js and a developer can start writing LESS right away: http://lesscss.org/#client-side-usage-browser-options - anything similar for cssnext?

Does cssnext support source maps? This is an important feature for any non-trivial project, as it lets a developer inspect how the rules in the source language are translated into the rules read by the browser. Support for source maps in Chrome Developer Tools makes a workflow much more efficient: https://developer.chrome.com/devtools/docs/css-preprocessors