JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="child1"></div>
  <div class="child2"></div>
</div>

CSS

.container{
    background-color: gray;
    position: relative;
    padding: 20px;
    height: 70%;
    /* uncomment this and will work as expected */
    /* height: 70px; */
}

.child1{
  width: 75%;
  display: inline-block;
  height: 100px;
}

.child2{
    background-color: green;
    width: 75%;
    float: right;
    height: 100%;
}