<!DOCTYPE html>
<html>
<body>
<video id="v" controls></video>
<script>
const v = document.getElementById("v");

function audioTrack() {
  const ctx = new AudioContext(), oscillator = ctx.createOscillator();
  const dst = oscillator.connect(ctx.createMediaStreamDestination());
  oscillator.start();
  return dst.stream.getAudioTracks()[0];
}

function videoTrack(width = 640, height = 480) {
  const canvas = Object.assign(document.createElement("canvas"), {width, height});
  canvas.getContext('2d').fillRect(0, 0, width, height);
  return canvas.captureStream(10).getVideoTracks()[0];
}

onload = () => v.srcObject = new MediaStream([videoTrack(), audioTrack()]);
</script>
</body>
</html>
