getUserMedia した MediaStream を MediaRecorder で WebM Stream　にしてから MediaSource に食わせて動画再生する。
Chrome60 以降ならば止まらずに連続再生できるようだ。
demo: https://jsfiddle.net/nthyfgvs/

const main = async(function* main(){
  const logging = true;
  let tasks = Promise.resolve(void 0);
  
  const devices = yield navigator.mediaDevices.enumerateDevices();
  console.table(devices);
  
  const stream = yield navigator.mediaDevices.getUserMedia instanceof Function ? navigator.mediaDevices.getUserMedia({video: true, audio: true}) :
                       navigator.getUserMedia instanceof Function ? new Promise((resolve, reject)=> navigator.getUserMedia({video: true, audio: true}, resolve, reject)) :
                       navigator.webkitGetUserMedia instanceof Function ? new Promise((resolve, reject)=> navigator.webkitGetUserMedia({video: true, audio: true}, resolve, reject)) :
                       Promise.reject(new Error("cannot use usermedia"));

  
  if(logging){
  	stream.addEventListener("active", (ev)=>{ console.log(ev.type); });
    stream.addEventListener("inactive", (ev)=>{ console.log(ev.type); });
    stream.addEventListener("addtrack", (ev)=>{ console.log(ev.type); });
    stream.addEventListener("removetrack", (ev)=>{ console.log(ev.type); });
  }
  
  const rec = new MediaRecorder(stream, {mimeType: 'video/webm; codecs="opus,vp8"'});
  if(logging){
  	rec.addEventListener("dataavailable", (ev)=>{ console.log(ev.type); });
  	rec.addEventListener("pause", (ev)=>{ console.log(ev.type); });
  	rec.addEventListener("resume", (ev)=>{ console.log(ev.type); });
  	rec.addEventListener("start", (ev)=>{ console.log(ev.type); });
    rec.addEventListener("stop", (ev)=>{ console.log(ev.type); });
    rec.addEventListener("error", (ev)=>{ console.error(ev.type, ev); });
  }
  
  const ms = new MediaSource();
  if(logging){
    ms.addEventListener('sourceopen', (ev)=>{ console.log(ev.type); });
    ms.addEventListener('sourceended', (ev)=>{ console.log(ev.type); });
    ms.addEventListener('sourceclose', (ev)=>{ console.log(ev.type); });
    ms.sourceBuffers.addEventListener('addsourcebuffer', (ev)=>{ console.log(ev.type); });
    ms.sourceBuffers.addEventListener('removesourcebuffer', (ev)=>{ console.log(ev.type); });
  }
  
  const video = document.createElement("video");
  if(logging){
    video.addEventListener('loadstart', (ev)=>{ console.log(ev.type); });
    video.addEventListener('progress', (ev)=>{ console.log(ev.type); });
    video.addEventListener('loadedmetadata', (ev)=>{ console.log(ev.type); });
    video.addEventListener('loadeddata', (ev)=>{ console.log(ev.type); });
    video.addEventListener('canplay', (ev)=>{ console.log(ev.type); });
    video.addEventListener('canplaythrough', (ev)=>{ console.log(ev.type); });
    video.addEventListener('playing', (ev)=>{ console.log(ev.type); });
    video.addEventListener('waiting', (ev)=>{ console.log(ev.type); });
    video.addEventListener('seeking', (ev)=>{ console.log(ev.type); });
    video.addEventListener('seeked', (ev)=>{ console.log(ev.type); });
    video.addEventListener('ended', (ev)=>{ console.log(ev.type); });
    video.addEventListener('emptied', (ev)=>{ console.log(ev.type); });
    video.addEventListener('stalled', (ev)=>{ console.log(ev.type); });
    video.addEventListener('timeupdate', (ev)=>{ console.log(ev.type); }); // annoying
    video.addEventListener('durationchange', (ev)=>{ console.log(ev.type); });
    video.addEventListener('ratechange', (ev)=>{ console.log(ev.type); });
    video.addEventListener('play', (ev)=>{ console.log(ev.type); });
    video.addEventListener('pause', (ev)=>{ console.log(ev.type); });
    video.addEventListener('error', (ev)=>{ console.warn(ev.type, ev); });
  }
  video.volume = 0;
  video.controls = true;
  video.autoplay = true;
  document.body.appendChild(video);
  
  yield new Promise((resolve, reject)=>{
    ms.addEventListener('sourceopen', ()=> resolve(), {once: true});
    //video.srcObject = ms;
    video.src = URL.createObjectURL(ms);
  });

  const sb = ms.addSourceBuffer(rec.mimeType);
  if(logging){
    sb.addEventListener('updatestart', (ev)=>{ console.log(ev.type); }); // annoying
    sb.addEventListener('update', (ev)=>{ console.log(ev.type); }); // annoying
    sb.addEventListener('updateend', (ev)=>{ console.log(ev.type); }); // annoying
    sb.addEventListener('error', (ev)=>{ console.error(ev.type, ev); });
    sb.addEventListener('abort', (ev)=>{ console.log(ev.type); });
  }

  const stop = async(function* stop(){
  	console.info("stopping");
  	if(sb.updating){ sb.abort(); }
    if(ms.readyState === "open"){ ms.endOfStream(); }
    rec.stop();
    stream.getTracks().map((track)=>{ track.stop(); });
    yield video.pause();
    console.info("end");
  });
  
  const button = document.createElement("button");
  button.innerHTML = "stop";
  button.addEventListener("click", ()=>{
    document.body.removeChild(button);
  	tasks = tasks.then(stop);
  }, {once: true});
  document.body.appendChild(button);

  let i = 0;
  rec.ondataavailable = ({data})=>{
    tasks = tasks.then(async(function*(){
      console.group(""+i);

      try{
        if(logging){ console.log("dataavailable", "size:", data.size); }

        if(data.size === 0){
          console.warn("empty recorder data");
          throw new Error("empty recorder data");
        }

        const buf = yield readAsArrayBuffer(data);

        yield new Promise((resolve, reject)=>{
          sb.addEventListener('updateend', ()=> resolve(), {once: true});
          sb.addEventListener("error", (ev)=> reject(ev), {once: true});
          sb.appendBuffer(buf);
        });

        if(logging){
          console.log("timestampOffset", sb.timestampOffset);
          console.log("appendWindowStart", sb.appendWindowStart);
          console.log("appendWindowEnd", sb.appendWindowEnd);
          for(let i=0; i<sb.buffered.length; i++){
            console.log("buffered", i, sb.buffered.start(i), sb.buffered.end(i));
          }
          for(let i=0; i<video.seekable.length; i++){
            console.log("seekable", i, video.seekable.start(i), video.seekable.end(i));
          }
          console.log("webkitAudioDecodedByteCount", video.webkitAudioDecodedByteCount);
          console.log("webkitVideoDecodedByteCount", video.webkitVideoDecodedByteCount);
          console.log("webkitDecodedFrameCount", video.webkitDecodedFrameCount);
          console.log("webkitDroppedFrameCount", video.webkitDroppedFrameCount);
        }

        if (video.buffered.length > 1) {
          console.warn("MSE buffered has a gap!");
          throw new Error("MSE buffered has a gap!");
        }
      }catch(err){
        console.error(err);
        yield stop();
        console.groupEnd(""+i); i++;
        return Promise.reject(err);
      }
      
      console.groupEnd(""+i);
      i++;
    }));
  };
  
  rec.start(1000);
  console.info("start");
});



function sleep(ms){
  return new Promise(resolve =>
    setTimeout((()=>resolve(ms)), ms));
}


function readAsArrayBuffer(blob) {
  return new Promise((resolve, reject)=>{
    const reader = new FileReader();
    reader.addEventListener("loadend", ()=> resolve(reader.result), {once: true});
    reader.addEventListener("error", (ev)=> reject(reader.error), {once: true});
    reader.readAsArrayBuffer(blob);
  });
}


function async(generatorFunc){
  return function (arg) {
    const generator = generatorFunc(arg);
    return next(null);
    function next(arg) {
      const result = generator.next(arg);
      if(result.done){ return result.value; }
      else if(result.value instanceof Promise){ return result.value.then(next); }
      else{ return Promise.resolve(result.value); }
    }
  }
}

main();
related issues
