CSS3 provide a simple way to make animation from one style to another without using flash or javascript. The easy way you do is add a transition attribute in the starting style. Below show a simple example:

#top_menu li a {
	color: #FFF;
	padding: 5px 10px;
	border-radius: 10px;
	text-decoration: none;

	-webkit-transition: all 300ms linear;
	-moz-transition: all 300ms linear;
	-o-transition: all 300ms linear;
}
#top_menu li a:hover {
	background-color: #F9C;
	color: #09F;
}

The result you can refer to the top menu on this page by using firefox or chrome.