<style type="text/css">
#yinyang {
 width: 100px; height: 50px;
 background: #fff;
 border-color: #000;
 border-style: solid;
 border-width: 2px 2px 50px 2px;
 border-radius: 100%;
 position: relative;
}
#yinyang:before {
 content: "";
 position: absolute;
 top: 50%;
 left: 0;
 background: #fff;
 border: 18px solid #000;
 border-radius: 100%;
 width: 14px;
 height: 14px;
}
#yinyang:after {
 content: "";
 position: absolute;
 top: 50%;
 left: 50%;
 background: #000;
 border: 18px solid #fff;
 border-radius:100%;
 width: 14px;
 height: 14px;
}
</style>

<div id="yinyang"></div>