JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="top_content"></div>
    <div id="remaining_content"></div>
</div>

CSS

#container {
    width: 400px;
    height: 400px;
    border: 5px solid black;
}
#top_content {
    background-color: blue;
    width: 100%;
    height: 50px;
}
#remaining_content {
    background-color: red;
    width: 100%;
    height: 100%;
}