SOURCE

    <div class="box">
        <div class="child"></div>
    </div>

        html, body {
            /* height: 100%; */
            margin: 0;
        }
        .box {
            padding: 0 10px;
            height: 100vh;
            background: #f00;
            line-height: 100vh;
        }
        .child {
            display: inline-block;
            width: 0;
            padding: 25% 50%;
            background: #ff0;
            margin-bottom: -25%;
        }