JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="first"></div>
    <div id="second"></div>
</div>

CSS

#container {
    height: 600px;
    background-color: #ccc;
}
#first {
    height: 60px;
    background-color: #ff3;
    float: left;
    width: 100%;
}

#second {
    background-color: #3ff;
    height: 100%;
}