<style>
div{
width:300px;
height:300px;
background:#89F;
margin:100px auto; transition:width 1s ease-out .5s;
-moz-transition:width 1s ease-out .5s;
-webkit-transition:width 1s ease-out .5s;
-o-transition:width 1s ease-out .5s;
}
div:hover{
width:600px;
}
</style> <div></div>