求大神指教为什么效果消失不出来
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>3D</title>
<style>
#block
{
width:400px;
height:400px;
background-color:blue;
margin:0 auto;
-o-transition:background-color 3s;
}
#block:hover{
background-color:red;
}
</style>
</head>
<body>
<div id="block">
</div>
</body>
</html>
为什么显示不出渐进效果
