// Youtube Videos
div.yt {
  position: relative;
  padding-bottom: 56.25%; /* 16:9, for an aspect ratio of 1:1 change to this value to 100% */
  iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
  }
}

// Other Videos
video {
  max-width: 100%;
}