Call el.offsetHeight; to force update of computed style.

chrome/browser/resources/print_preview/print_preview_animations.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Counter used to give webkit animations unique names.
 var animationCounter = 0;
 
 function addAnimation(code) {
   var name = 'anim' + animationCounter;
   animationCounter++;
@@ skipping 46 matching lines @@
  * Fades out an element. Used for both printing options and error messages
  * appearing underneath the textfields.
  * @param {HTMLElement} el The element to be faded out.
  */
 function fadeOutElement(el) {
   if (!el.classList.contains('visible'))
     return;
   el.style.height = 'auto';
   var height = el.offsetHeight;
   el.style.height = height + 'px';
-  var animName = addAnimation('');
+  el.offsetHeight;  // Should force an update of the computed style.

[Noam Samuel, 2014/01/15 22:08:44]

Should this have a TODO attached to it to remove it when the corresponding bug
in Blink is fixed?

   var eventTracker = new EventTracker();
   eventTracker.add(el, 'webkitTransitionEnd',
-                   onFadeOutTransitionEnd.bind(el, animName, eventTracker),
+                   onFadeOutTransitionEnd.bind(el, eventTracker),
                    false);
   el.classList.add('closing');
   el.classList.remove('visible');
 }
 
 /**
  * Executes when a fade out animation ends.
- * @param {string} animationName The name of the animation to be removed.
  * @param {EventTracker} eventTracker The |EventTracker| object that was used
  *     for adding this listener.
  * @param {WebkitTransitionEvent} event The event that triggered this listener.
  * @this {HTMLElement} The element where the transition occurred.
  */
-function onFadeOutTransitionEnd(animationName, eventTracker, event) {
+function onFadeOutTransitionEnd(eventTracker, event) {
   if (event.propertyName != 'height')
     return;
-  fadeInOutCleanup(animationName);
   eventTracker.remove(this, 'webkitTransitionEnd');
   this.hidden = true;
 }
 
 /**
  * Executes when a fade in animation ends.
  * @param {EventTracker} eventTracker The |EventTracker| object that was used
  *     for adding this listener.
  * @param {WebkitAnimationEvent} event The event that triggered this listener.
  * @this {HTMLElement} The element where the transition occurred.
@@ skipping 65 matching lines @@
     while (el.childNodes.length > 0)
       div.appendChild(el.firstChild);
     el.appendChild(div);
   }
 
   div.className = '';
   div.classList.add('collapsible');
   for (var i = 0; i < classes.length; i++)
     div.classList.add(classes[i]);
 }