html {
    font-size: 100%;
    font-family: var(--bs-font-sans-serif);
}

.ratio-9x16 {
    position: relative;
    width: 100%;
    padding-top: 177.77%; /* 16/9 * 100% = ~177.77% */
}

    .ratio-9x16 iframe {
        position: absolute;
        top: 0;
        left: 0;
        width: 100%;
        height: 100%;
        border: 0;
    }