<!DOCTYPE html>
<style>
.container {
    -webkit-filter: drop-shadow(5px 5px 5px black);
}
.circle-mask {
    border-radius: 80px;
    overflow: hidden;
    width: 100px;
    height: 100px;
}
.green-box {
    width: 100px;
    height: 100px;
    background-color: green;
}
</style>
<div class="container">
  <div class="circle-mask">
    <div class="green-box"></div>
  </div>
</div>
