HTML

html iframe aspect ratio

// HTML
<div class="video_wrapper">
<iframe frameborder="0" width="100%" height="100%" src="https://www.dailymotion.com/embed/video/Your_dailymotion_embed_URL" allowfullscreen allow="autoplay">
</iframe></div>

// CSS
.video_wrapper {
  position: relative;
  padding-bottom: 56.25%; /* 16:9 */ 
}
iframe{
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}