User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0b7pre) Gecko/20100924 Firefox/4.0b7pre Build Identifier: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0b7pre) Gecko/20100924 Firefox/4.0b7pre An element node which has set a background-color style, shows a black overlay box while applying opacity via JS to the element in question. This happens all the time in Mac OS X only. Reproducible: Always Steps to Reproduce: 1. Go to http://jsfiddle.net/w00fz/3kmHU/ 2. Click "Run" 3. See the Results container. Actual Results: The first 3 lines will fade with a black overlay during the whole transition and then disappear. The last 3 lines will fade normally as expected, because background-color is transparent. Expected Results: All 6 lines should act like the last 3, wether the background-color is solid or transparent. This is not JS Framework related, nor to animations. Here is a simple setStyle example with no animations: http://jsfiddle.net/w00fz/3kmHU/1/ And here http://jsfiddle.net/w00fz/3kmHU/1/show/ you can try the following code to reproduce in a non JS Framework environment: var results = document.querySelectorAll('div'); for (var i = 0, l = results.length; i < l; i++){ results[i].style.opacity = 0.2; }