如何使用MediaRecorder（）API下载<video>标签的内容？

Question

我正在尝试记录<video>标记的内容并以.webm格式下载文件。这是我的代码：

return new Promise(async resolve => {
  const stream = await navigator.mediaDevices.getUserMedia({
    video: true,
    audio: true
  });

  const mediaRecorder = new MediaRecorder(document.getElementById('#my-video-tag').captureStream());
  const audioChunks = [];

  mediaRecorder.addEventListener('dataavailable', event => {
    audioChunks.push(event.data);
  });

  const start = () => mediaRecorder.start();

  const stop = () =>
    new Promise(resolve2 => {
      mediaRecorder.addEventListener('stop', () => {
        const audioBlob = new Blob(audioChunks);
        resolve2({ audioBlob });
      });

      mediaRecorder.stop();
    });

  resolve({ start, stop });
});

这是问题所在：

const mediaRecorder = new MediaRecorder(document.getElementById('#my-video-tag').captureStream());

以上内容仅从video标签下载1帧，因此基本上是静止图像。如何使其正常工作，以便下载整个视频？