Have you ever heard of CSS counters? It’s a feature of CSS for numbering non-consecutive items, wherever they might be in the DOM. It doesn’t have a lot of uses cases but it’s important to understand this kind of new features to understand how powerful is CSS today.
These are two articles that I have selected for you:
http://www.sitepoint.com/understanding-css-counters-and-their-use-cases/
http://codersblock.com/blog/fun-times-with-css-counters/
Trick of the day: CSS counters