Responsive video_embed_field – Simple markup as an alternative for https://www.drupal.org/project/fitvids

responsive_video_embed_field.css

.embedded-video .player {
  position: relative;
  padding-bottom: 56.25%;
  height: 0;
  overflow: hidden;
  max-width: 100%;
}

.embedded-video .player iframe,
.embedded-video .player object,
.embedded-video .player embed {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}