<!doctype html>
<html>
<head>
<style>
.round01{
42px;
height: 42px;
border-radius: 50px;
position: relative;
background:#000;
transition:background 2s;
/*transition*属性值：过度时间*/
}
.round01:hover{background: red;}
}
</style>
</head>
<body>
<h1>transition属性说明</h1>
<div class="round01">
</div>
</html>