<div class="circle"></div>

.circle {
  height: 200px;
  width: 200px;
  border-radius: 50%;
  background-color: #3d9970;
}

body, html {
  height: 100%;
}

body {
  padding: 0;
  background-color: #333;
  display: flex;
  align-items: center;
  justify-content: center;
}