HTML

<div class="bg-video">
  <video class="bg-video__content" autoplay muted loop>
    <source src="img/video.mp4" type="video/mp4" />
    <source src="img/video.webm" type="video/webm" />
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  </video>
</div>

CSS

.bg-video {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  z-index: -1;
  opacity: 0.15;
}

.bg-video__content {
  height: 100%;
  width: 100%;
  object-fit: cover; // background-size: cover 와 비슷함. (HTML 요소 or 비디오와 작동)
  
}

적용하여 만든 예시