新月形状

box-shadow做的

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

</style>
<div class="center"></div>