Here are a couple quotes from https://developer.chrome.com/articles/css-nth-child-of-s/#pr... :

New in CSS Selectors Level 4 is the ability to optionally pass a selector list into :nth-child() and :nth-last-child().

For example, :nth-child(2 of .highlight) selects the second matching element that has the .highlight class. Put differently: out of all children with the class .highlight, select the second one.