Edit in JSFiddle

#box {
    margin: 0 auto;
    width: 100px;
    height: 100px;
    background-color: #ffcc00;
    -webkit-transition: width 1s ease-in-out 0.5s;
    transition: width 1s ease-in-out 0.5s;
}

#box:hover {
    width: 300px;
}

<div id="container">
    <div id="box"></div>
</div>