/* Helper function to add an event listener.
 * @param el The element we want to add an event listener to.
 * @param event The event we want to listen to.
 * @param callback The callback function to call when the event is transmitted.
 */
function addEvent(el, event, callback) {
  if ('addEventListener' in el) {
    el.addEventListener(event, callback, false);
  } else {
    el['e' + event + callback] = callback;
    el[event + callback] = function() {
      el['e' + event + callback](window.event);
    };
    el.attachEvent('on' + event, el[event + callback]);
  }
}

function clearWarningOnInput(inputId, warningId) {
  var el= document.getElementById(inputId);
  addEvent(el, 'input', function() {
    var cl = document.getElementById(warningId).classList;
    if (!cl.contains('hidden-warning')) {
      cl.add('hidden-warning');
    }
  });
}

/* Helper function to add an event listener.
 * @param {Element} el - The element we want to add an event listener to.
 * @param {Event} event - The event we want to listen to.
 * @param {function} callback - The callback function to call when the event is
 * transmitted.
 */
function addEvent(el, event, callback) {
  if ('addEventListener' in el) {
    el.addEventListener(event, callback, false);
  } else {
    el['e' + event + callback] = callback;
    el[event + callback] = function() {
      el['e' + event + callback](window.event);
    };
    el.attachEvent('on' + event, el[event + callback]);
  }
}

function clearWarningOnInput(inputId, warningId) {
  var el= document.getElementById(inputId);
  addEvent(el, 'input', function() {
    var cl = document.getElementById(warningId).classList;
    if (!cl.contains('hidden-warning')) {
      cl.add('hidden-warning');
    }
  });
}

Change doc style in addEvent()