程式好好玩

CSS

*{
  margin:0;
}
video {
  width:100vw;
  height:100vh;
  object-fit:cover;
  z-index:-1;
  position:absolute;
}

HTML

<video id="video" autoplay></video>

JavaScript

var video = document.getElementById('video');

// Get access to the camera!
if(navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
  // Not adding `{ audio: true }` since we only want video now
  navigator.mediaDevices.getUserMedia({ video: true }).then(function(stream) {
    video.src = window.URL.createObjectURL(stream);
    video.play();
  });
}

var video = document.getElementById('video');

DEMO (用手機瀏覽)

https://thimbleprojects.org/pochao/354214/