/* After: https://stackoverflow.com/questions/17838607/making-an-iframe-responsive */
.youtube {
    position: relative;
    padding-bottom: 56.25%; /* 16:9 */
    padding-top: 25px;
    height: 0;
}
.youtube iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}