.video-block {
  text-align: center;
  margin-bottom: 25px;
}

@media (max-width: 640px) {

  .video-block {
      position: relative;
      padding-bottom: 56.25%;
      padding-top: 30px;
      height: 0;
      overflow: hidden;
  }
  .video-block iframe {
      position: absolute;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
  }

}