Spec is in "last call working draft": http://www.w3.org/TR/2014/WD-css-flexbox-1-20140325/#flex-containers
Value is ```display:flex```
Browser support is good in all but IE: http://caniuse.com/#feat=flexbox
What I did degrades nicely pre-IE10 (just stacks items vertically).
The scss mixins I added are the best polyfill I could find (-webkit, -ms, -moz prefixes, based on http://dev.opera.com/articles/advanced-cross-browser-flexbox/)