<div class="example"></div>

* {
  box-sizing: border-box;
}
body {
  min-height: 100vh;
  margin: 0;
  padding: 0;
  display: flex;
  justify-content: center;
  align-items: center;
}
.example {
  width: 100px;
  border-bottom: 20px solid #a0e2c2;
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;
}