<div class="trapezium"></div>

.trapezium {
  border-bottom: 100px solid #333;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  height: 0;
  width: 100px;
}

body, html {
  height: 100%;
}

body {
  background-color: #3d9970;
  display: flex;
  align-items: center;
  justify-content: center;
}