better navigation.onLine: serverReachable()

serverReachable.js

function serverReachable() {
  // Cross-browser XHR creation
  var request = new ( window.ActiveXObject || XMLHttpRequest )( "Microsoft.XMLHTTP" ),
      status_code;

  request.open(
    // requesting the headers is faster, and just enough
    "HEAD",
    // append a random string to the current hostname,
    // to make sure we're not hitting the cache
    "//" + window.location.hostname + "/?rand=" + Math.random(),
    // make a synchronous request
    false
  );
  try {
    request.send();
    status_code = request.status;
    // Make sure the server is reachable
    return ( status_code >= 200 && status_code < 300 || status_code === 304 );
  // catch network & other problems
  } catch (e) {
    return false;
  }
}