CSS Regions allow content to flow across multiple areas of the display, called regions. The beauty of CSS Regions is that you can separate the content from the layout. And with that comes the ability to create some responsive content very easily.
Update August 29, 2013:
I have a new post describing Cross-Browser Responsive Content Using CSS Regions. I describe there how you can do exactly what is described here, but also use the CSS Regions polyfill to get CSS Regions support on many other browser and devices.