.youtube-stc-wrapper {
    position: relative;
    padding-bottom: 56.25%; /*アスペクト比 16:9の場合の縦幅*/
    height: 0;
    overflow: hidden;
    margin-bottom: 20px;
}

.youtube-stc-wrapper iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}