<video style="width:800px;height: 800px;"></video>

<script>
  // 想要获取一个最接近 1280x720 的相机分辨率
  let constraints = { audio: true, video: { width: 1280, height: 720 } };
  navigator.mediaDevices.getUserMedia(constraints)
    .then(function(mediaStream) {
      let video = document.querySelector('video');
      video.srcObject = mediaStream;
      video.onloadedmetadata = function(e) {
        video.play();
      };
    })
    .catch(function(err) { console.log(err.name + ": " + err.message); }); // 总是在最后检查错误
</script>