JSFiddle - React, Tailwind, and code Playground

HTML

<div class="div div1">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Reprehenderit laborum, porro ex optio quibusdam atque voluptatibus voluptatem hic, beatae totam eveniet at, fuga ipsa officia voluptatum mollitia. Molestiae, sit, eum.</div>
<div class="div div2">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Reprehenderit laborum, porro ex optio quibusdam atque voluptatibus voluptatem hic, beatae totam eveniet at, fuga ipsa officia voluptatum mollitia. Molestiae, sit, eum.</div>

CSS

* {
  box-sizing: border-box;
}
.div1 {
  float: left;
  width: 300px;
  background: #eee;
}

.div2 {
  width: 100%;
  padding-left: 310px;
  background: #333;
  color: #fff;
}

@media screen and (max-width: 768px) {
  .div1 {
    float: none;
    width: 100%;
  }
  .div2 {
    padding-left: 0;
  }
}