<div class="wrap">
  <div class='main'></div>
</div>

.wrap {
  display: flex;
  justify-content: center;
  margin: 40px auto 0;
}
.main {
  background: #0aaaa0;
  border-radius: 50%;  /*等价于 border-radius: 150px / 100px; */
  width: 300px;
  height: 200px;
}