JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrap_top">
    <div class="wrap">
        <div class="col1">123</div>
        <div class="col2">456</div>
        <div style="clear:both;"></div>
    </div>
</div>

CSS

.wrap_top {
    background: transparent;
    position: relative;
}

.wrap_top:after {
    background: #e1e1e1;
    content: '';
    display: block;
    position: absolute;
    top: 0; right: 0; bottom: 0; left: 70%;
    z-index: -1;
}

.wrap {
    margin: 0 auto;
    border: 1px dashed #252525;
    width: 920px;
    min-height: 300px;
    z-index: 999;
}

.col1 {
    background: #f1f1f1;
    width: 600px;
}

.col2 {
    background: #e1e1e1;
    width: 320px;
}

.col1, .col2 {
    min-height: 300px;
    float: left;
}