.ifrn{
  width: 640px;
  height: 480px;
}

@media screen and (max-width: 640px) {
 .ifrm-wrapper {
    position: relative;
    width: 100%;
    padding: calc(480/640*100%) 0 0;
  }
  .ifrm{
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
  }
}