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function css(element, property) {
  return window.getComputedStyle(element, null).getPropertyValue(property);
}

window.onload = function () {
  var span = document.createElement('span');
  
  span.className = 'fa';
  span.style.display = 'none';
  document.body.insertBefore(span, document.body.firstChild);
  
  if ((css(span, 'font-family')) !== 'FontAwesome') {
    // add a local fallback
  }
  document.body.removeChild(span);
};