<div class="pentagon"></div>

.pentagon {
    position: relative;
    width: 54px;
    border-width: 50px 18px 0;
    border-style: solid;
    border-color: #01ff70 transparent;
}
.pentagon:before {
    content: "";
    position: absolute;
    height: 0;
    width: 0;
    top: -85px;
    left: -18px;
    border-width: 0 45px 35px;
    border-style: solid;
    border-color: transparent transparent #01ff70;
}

body, html {
  height: 100%;
}

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