<video id="webcam"></video>

<script type="text/javascript">
  function onSuccess(stream) {
    var video = document.getElementById('webcam');
    if (window.URL) {

      try{
        video.src = window.URL.createObjectURL(stream);
      }catch (e){
        console.log(e);
        video.srcObject = stream;
      }
       
  } else {
    video.src = stream;
  }

  video.autoplay = true; 
  // 或者 video.play();
  }

if (navigator.getUserMedia) {
  navigator.getUserMedia({video:true}, onSuccess,onError);
} else {
  document.getElementById('webcam').src = 'somevideo.mp4';
}

function onError(error) {
  console.log("navigator.getUserMedia error: ", error);
}
</script>