JSFiddle - React, Tailwind, and code Playground

HTML

<section id="section">
  <div class="leftbox">lol</div>
  <div class="rightbox">lol</div>
  <div class="Soubox">lol</div>
</section>

CSS

#section
{
  display: flex;
  flex-wrap:wrap;
}


.leftbox
{background-color: red; width: 50%}

.rightbox
{background-color: blue; width: 50%}

.Soubox
{background-color: green; width: 100%;}