css nth-child
/* Select the first list item */
li:nth-child(1) { }
/* Select odd list items */
li:nth-child(odd) { }
/* Select even list items */
li:nth-child(even) { }
/* Select each list item every 3 (3,6,9...) */
li:nth-child(3n) { }
/* Select each list item every 3 starting from the fourth (4,7,10...) */
li:nth-child(3n+1) { }