.ratio-9x16 {
    position: relative;
    width: 100%;
    padding-bottom: 177.78%; /* (9 / 16) * 100 = 177.78% */
    background-color: #000;
}
.ratio-9x16 iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}