Steps to reproduce: 1. Go to http://jqueryui.com/demos/addClass/ 2. Once the page has finished loading, click "Run Effect" in the center box Expected outcome: The gray box gradually expands and its text becomes larger (view the page in Firefox 3.6 to see the intended behavior) and, after a few seconds, it returns to normal size. Actual outcome: Nothing happens. Same outcome in Chrome and Safari 4.0.x and 3.0.x (except possibly 3.0.10), as well as Firefox 3.5. Opera 10 performs the animation but messes it up pretty badly.