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And this awesome animation explains how it works: http://codepen.io/chriscoyier/details/lotjh

This is not pure CSS. This requires Markup, and Markup is something that Marks Up Text to give it semantic meaning.

Typically you use :before or :after in CSS to add the required content.

I mean, it could be no-classes-pure css if we just had divs with no classes and then used n-th child selectors, but then what is pure css?

this is what I was referring to

That's so awesome! I wish http://thecodeplayer.com/ was more active for things like this.