JSFiddle - React, Tailwind, and code Playground

HTML

<div id = "wrapper">
    <div class="header">Header</div>
    <div>
        <div class="leftpanel">Left Panel</div>
        <div class="rightpanel">Right Panel</div>
    </div>
</div>

CSS

* {
    margin: 0;
    padding: 0;
    border: 0;
}

html, body {
    height: 100%;
}

.header{
    background: #333;
    padding: 15px;
    text-align:center;
    font-size: 18px;
    font-family: sans-serif;
    color: #fff;
}

.leftpanel{
    background: #CCC;
}

.rightpanel{
    background: #666;
}

#wrapper {
    height: 100%;
    display: -webkit-flex;
    display: flex;
    -webkit-flex-direction: column;
    flex-direction: column;
    outline: 1px solid red;
}

#wrapper > .header {
    -webkit-flex: 0 0 auto;
    flex: 0 0 auto;
}

#wrapper > .header + div {
    -webkit-flex: 1 1 auto;
    flex: 1 1 auto;
    display: -webkit-flex;
    display: flex;
    -webkit-flex-direction: row;
    flex-direction: row;
}

#wrapper > .header + div > div:first-of-type {
    -webkit-flex: 7 0 0;
    flex: 7 0 0;
}

#wrapper > .header + div > div:last-of-type {
    -webkit-flex: 3 0 0;
    flex: 3 0 0;
}