JSFiddle - React, Tailwind, and code Playground

HTML

<div id="header"></div>
<div class="left"></div>
<div class="right"></div>
<div id="footer"></div>

CSS

#header {
    height: 70px;
    /*width: 100%;*/
    margin-top: -10px;
    margin-bottom: 5px;
    margin-right: 50px;
    margin-left: 50px;
    background-color: gray;
    border-radius: 0px 0px 10px 10px;
    /*position: fixed;*/
    z-index: 1;
    margin-bottom: -60px; /* <-- */
}

.left {
    display: inline-block;
    height: 200px;
    width: 10%;
    margin-top: 65px;
    background-color: gray;
    border-radius: 5px;
    float: left;
}

.right {
    display: inline-block;
    height: 200px;
    width: 89%;
    margin-top: 65px;
    margin-bottom: 5px;
    background-color: gray;
    border-radius: 5px;
    float: right;
}

#footer {
    height: 70px;
    background-color: gray;
    border-radius: 5px;
    clear: both;
}