:nth-child()

The :nth-child(an+b) pseudo-class notation represents an element that has an+b-1 siblings before it in the document tree, for any positive integer or zero value of n. 6.6.5.2. :nth-child() pseudo-class

Links about :nth-child()

1. :nth Tester

   by CSS Tricks

   

2. Mastering the :nth-child | CSS3 pseudo classes and :nth-child ranges

   by James Menera

   

3. :nth-child()

   posted on April 3, 2020 by MDN

   The :nth-child() CSS pseudo-class matches elements based on their position in a group of siblings.

   

Code sample

li:nth-child(2n) {
  border: 5px solid fuchsia;
}