The Flexbox css spec allows for more adjustable layouts. The flex-directionproperty allows you to easily change the layout on the children of an element without making any changes to the dom, which is particularly useful when combined with media queries.
Idea is when greater then 599px, it will show in row layout, when it goes down under 599px, it will show in column layout.
[CSS3] Using flex-direction to layout content horizontally and vertically
原文：http://www.cnblogs.com/Answer1215/p/5452360.html