JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

html, body{
    height: 100%;
    margin: 0;
}
.header{
    background: #333;
    padding: 15px;
    text-align:center;
    font-size: 18px;
    font-family: sans-serif;
    color: #FFF;
}
.leftpanel, .rightpanel{
    height: 100%;
}
.leftpanel{
    float: left;
    width: 70%;
    background: #CCC;
}
.rightpanel{
    float: left;
    width: 30%;
    background: #666;
}
.clearfix{clear: both}