JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div class="table">
        <div class="table-cell">
            this normally has 100%
        </div>
        <div class="table-cell">
            <div>
                this should get height 100% too
            </div>
        </div>
    </div>
</body>

CSS

.table {
    display: table;
    height: 100%;
}
.table > div:first-child {
    height: 300px;
    background: #f00;
}
.table-cell {
    display: table-cell;
    vertical-align: top;
    height: 100%;
}
.table-cell > div {
    height: 100%;
    background: #0c0;
}