(function() {
  let canvas = document.querySelector('canvas');
  // Optional frames per second argument.
  let stream = canvas.captureStream(25);
  var options = {mimeType: 'video/webm; codecs=vp9'};
  let recorder = new MediaRecorder(stream, options);
  let blobs = [];

  function download(blob) {
    var url = window.URL.createObjectURL(blob);
    var a = document.createElement('a');
    a.style.display = 'none';
    a.href = url;
    a.download = 'test.webm';
    document.body.appendChild(a);
    a.click();
    setTimeout(function() {
      document.body.removeChild(a);
      window.URL.revokeObjectURL(url);
    }, 100);
  }

  recorder.ondataavailable = e => { console.log(e.data); if (e.data && e.data.size > 0) blobs.push(e.data)};
  recorder.onstop = (e) => download(new Blob(blobs, {type: 'video/webm'}));

  recorder.start(10); // collect 10ms chunks of data

  // Record for 10 seconds.
  setTimeout(()=> recorder.stop(), 10000);
})();