<div class="triangle"></div>

.triangle {
  height: 0;
  width: 0;
  border-left: 100px solid #ff4136;
  border-right: 100px solid transparent;
  border-bottom: 100px solid transparent;
  border-top: 100px solid transparent;
}

body, html {
  height: 100%;
}

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