Set*_*eth 35
<div class="doughnut"></div>
.doughnut { 
    border: 50px solid #f00;
    border-radius: 100px;
    height:100px;
    width:100px;
}
div{width:200px; height:200px; border:1px solid black; position:relative; border-radius:200px;}
div:before{content:''; width:50px; height:50px; display:block; position:absolute; top:75px; left:75px; border:1px solid black; border-radius:200px;}
这个形状也可以用 css3 绘制radial-gradient():
div {
  background: radial-gradient(circle, transparent 40%, purple 40%);
}
div {
  background: radial-gradient(circle, transparent 40%, purple 40%);
}
body {
  background: linear-gradient(orange, red) no-repeat;
  min-height: 100vh;
  margin: 0;
}
div {
  background: radial-gradient(circle, transparent 40%, purple 40%);
  border-radius: 100%;
  height: 300px;
  width: 300px;
  margin: 25px;
}