box-shadow做的
<style>
.center
{
position: absolute;
margin: auto;
top: 0;
right: 0;
bottom: 0;
left: 0;
100px;
height: 100px;
background-color: transparent;
border-radius: 50%;
box-shadow: 25px 10px 0px 0px BLUE;
}
</style>
<div class="center"></div>