JSFiddle - React, Tailwind, and code Playground

HTML

<div id="status1">
 
</div>
<div id="status2">
 
</div>

JavaScript

var link = document.createElement('link');

link.setAttribute("href", "https://webkit.org/wp-content/themes/webkit/style.css");
link.setAttribute('rel', 'stylesheet');

var result = {
  scriptFinished: false,
  onErrorCalled: false
}

link.onerror = function() {
	result.onErrorCalled = true;
  if (!result.scriptFinished) {
    document.getElementById('status1').innerText += 'onerror called immediately';
  } else {
    document.getElementById('status1').innerText += 'success';
  }
}

document.body.appendChild(link);

result.scriptFinished = true;

setTimeout(function() {
  if (!result.onErrorCalled) {
    document.getElementById('status2').innerText += 'onerror was not called';
  } else {
    document.getElementById('status2').innerText += 'onerror called';
  }
}, 300);