JSFiddle - React, Tailwind, and code Playground

HTML

<div id="_1">
     1
    <div id="_2">3</div>
  </div>
  <div id="_3">3
    <div id="_4">4
      <div id="_5">5</div>
      <div id="_6">6</div>
    </div>
  </div>

CSS

div:last-child { border: 1px #f00 solid; }