css - css coding that I have never seen

Question

I am studying a free css template called zerofour found at http://html5up.net/zerofour/, and running across a css coding that I have never seen before. In the HTML, some div

Answer 1

html5up.net uses a framework called skel to make the layouts responsive.

If your question is "how do I make sense of these classes" rather than "how do these escape sequences work", then the reference you're looking for is at https://github.com/n33/skel/blob/master/docs/skel-layout.md#usage-1.