<!DOCTYPE html>
<html>
<head>
<style>
div
{
width:100px;
height:100px;
background:blue;
transition-property: width;
transition-duration: 2s;
-moz-transition-property: width; /* Firefox 4 */
-moz-transition-duration: 2s; /* Firefox 4 */
-webkit-transition-property: width; /* Safari and Chrome */
-webkit-transition-duration: 2s; /* Safari and Chrome */
-o-transition-property: width; /* Opera */
-o-transition-duration: 2s; /* Opera */
}
div:hover
width:300px;
</style>