JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="header">
    
    </div>
    <div id="mainContent">
    
    </div>
</div>

CSS

body, html {
    margin: 0;
    height: 100%;
}

#container {
    position: relative;
    width: 100%;
    height: 100%;
}

#header {
    height: 150px;
    width: 100%;
    background-color: #999999;
}

#mainContent {
    width: 760px;
    top: 150px;
    bottom: 0;
    right: 0;
    left: 0;
    background-color: #CCCCCC;
    margin: 0 auto;
    position: absolute;
}