.iframe-container {
  width: 100%;
  height: 0;
  overflow: hidden;

  /* element ratio is 4:3 (3/4 * 100) */
  padding-top: 75%;
}

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