title: When does XHR `ontimeout` is possibly called when `timeout` is 0?
body: <p>Is there is any conditions that could lead to <code>ontimeout</code> triggered when <code>timeout</code> is set to 0?</p>
  <pre><code>var xhr = new XMLHttpRequest();
  xhr.open('POST', '/something', true);
  
  xhr.timeout = 0; // No timeout
  
  xhr.ontimeout = function (e) {
      // Any circumstances to get here?
  };
  
  xhr.send(...);
  </code></pre>
  <p>Some sort of slow connection? Or very long internal browser connect/read timeout?</p>
  <p>I found some users of the site I maintain fall into this problem and want to figure out why.</p>