9591308a__info___Pseudo_class_selectors_CSS_Pseudo_class_selectors_Purpose
[Pseudo-class selector] :first-child [Purpose] 
Selects the first child element of a markup sequence. For example, ul li:first-child would specify that the style declaration applies to the first li element in any unordered list.
demosthenes.info – Pseudo-class selectors
Purpose