用css3实现一个带缺口的圆圈（图）

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
    .box{
        margin:300px;
        position:relative;
    }
    .box1 {
            width:100px;
            height:100px;
            background-color:transparent;
            border:3px solid #F00;
            border-radius:60px;
            margin-top:20px;
        }
        .box2 {
            width:60px;
            height:60px;
            background-color:#fff;
            z-index:1;
            border-radius:45px;
            position:absolute;
            top:0;
            left:80px;
        }
    </style>
</head>
<body>
<div class="box">
    <div class="box1"></div>
    <div class="box2"></div>
</div>
</body>
</html>

效果图：