.yt-wrapper {
    position: relative;
    width: 100%;
    max-width: 640px;
    margin: 0 auto;
    padding-top: 56.25%;
    /* 16:9 = 9/16≈0.5625 */
}

.yt-wrapper iframe {
    position: absolute;
    inset: 0;
    width: 100%;
    height: 100%;
    border: 0;
}