JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box left"></div>
<div class="box right"></div>

CSS

.box {
  float: left;
  width: 50%;
  height: 300px;
}
.left {
  background: red;
}
.right {
  background: green;
}


@media (max-width: 400px) {
  .box {
    width: 100%;
  }
}