Try it Yourself - CSS Reference transition-duration

<!DOCTYPE html>
<html>
<head>
<style>
div {
   width: 100px;
   height: 100px;
   background: red;
   -webkit-transition-property: width; /* Safari */
   -webkit-transition-duration: 5s; /* Safari */
   transition-property: width;
   transition-duration: 5s;
}

div:hover {
   width: 300px;
}
</style>
</head>
<body>

<p><b>Note:</b> This example does not work in Internet Explorer 9 and earlier versions.</p>

<div></div>

<p>Hover over the div element above, to see the transition effect.</p>

</body>
</html>