JSFiddle - React, Tailwind, and code Playground

HTML

<b>A</b><b>B</b><b>C</b><b>D</b><b>E</b><b>F</b><b>G</b><b>H</b><b>I</b>

CSS

b:nth-of-type(2n) {
  background: tomato;
}

b:nth-of-type(2n):after {
  display: block;
  content: "";
  clear: both;
}