[html] move div with CSS transition

Just to add my answer, it seems that the transitions need to be based on initial values and final values within the css properties to be able to manage the animation.

Those reworked css classes should provide the expected result :

_x000D_
_x000D_
.box{_x000D_
    position: relative;  _x000D_
    top:0px;_x000D_
    left:0px;_x000D_
    width:0px;_x000D_
}_x000D_
_x000D_
.box:hover .hidden{_x000D_
    opacity: 1;_x000D_
     width: 500px;_x000D_
}_x000D_
_x000D_
.box .hidden{    _x000D_
   background: yellow;_x000D_
    height: 300px;    _x000D_
    position: absolute; _x000D_
    top: 0px;_x000D_
    left: 0px;    _x000D_
    width: 0px;_x000D_
    opacity: 0;    _x000D_
    transition: all 1s ease;_x000D_
}
_x000D_
<div class="box">_x000D_
_x000D_
    <a href="#">_x000D_
        <img src="http://farm9.staticflickr.com/8207/8275533487_5ebe5826ee.jpg"></a>_x000D_
        <div class="hidden"></div>_x000D_
_x000D_
</div>
_x000D_
_x000D_
_x000D_

http://jsfiddle.net/u2FKM/2199/