JSFiddle - React, Tailwind, and code Playground

HTML

<div class="unten">unten</div>
<div class="oben">oben</div>

CSS

.unten{
	float: left;
	width: 400px;
	margin-right: 15px;
  background-color: #ccc;
}
.oben{
  background-color: red;
}
@media only screen and (max-width: 450px){
  .unten{
    float: none;
    width: 100%;
    box-sizing: border-box;
  }
}