div {
width:100px;
height:100px;
background:blue;
transition-property:width;
transition-duration:5s;
/* Firefox 4 */
-moz-transition-property:width;
-moz-transition-duration:5s;
/* Safari and Chrome */
-webkit-transition-property:width;
-webkit-transition-duration:5s;
/* Opera */
-o-transition-property:width;
-o-transition-duration:5s;
}

div:hover {
width:300px;
}