hugo-theme-wondermod/assets/css/extended/youtube.css

.youtube-embed-div {
    position: relative;
    padding-bottom: 56.25%;
    margin-bottom: 20px;
    height: 0;
    overflow: hidden;
}

.youtube-embed-frame {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border: 0;
}