JSFiddle - React, Tailwind, and code Playground

HTML

<div class = "main">
     Header
</div>
<div class="left">
    Bottom Div
</div>

CSS

* {
    -webkit-box-sizing:border-box;
    -moz-box-sizing:border-box;
    box-sizing:border-box; 
}
html, body {
    height:100%;
} 
body {
    padding:60px 0 0 0; /* 60 — header height*/
    margin:0;
}
.main,
.left {
    border:1px solid #000;
}
.main {
    width:100%;
    height:60px;
    margin-top: -60px;  /* 60 — header height*/
}
        
.left {
    height: 100%;
    width: 300px;
}