humhub/static/less/oembed.less

.oembed_snippet[data-oembed-provider="youtube.com"],
.oembed_snippet:not([data-oembed-provider]){
    margin-top: 10px;
    position: relative;
    padding-bottom: 55%;
    padding-top: 15px;
    height: 0;
    overflow: hidden;
}

.oembed_snippet iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}