<div class="wrap">
  <video autoplay muted loop playsinline>
    <source src="https://previews.customer.envatousercontent.com/h264-video-previews/63d3e475-cd5d-4251-bf47-bf256bd0e441/51338459.mp4" media="(max-width: 1000px)" />
    <source src="https://previews.customer.envatousercontent.com/h264-video-previews/eb992654-6121-40c0-890f-70e75afc2556/51341778.mp4" media="(min-width: 1000px)" />
    I'm sorry; your browser doesn't support HTML video.
  </video>
</div>

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

.wrap {
  position: relative;
  display: flex;
  width: 100%;
  height: 100vh;
}

video {
  position: absolute;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  object-fit: cover;
}