JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
      <li class="a">1st</li>
      <li class="b">2nd</li>
      <li>3rd</li>
      <li class="b">4th</li>
      <li class="a">5th</li>
    </ul>

CSS

.a + .b {
      background-color: #ff0000;
    }