<div></div>

div {
  width: 350px;
  height: 350px;
  background-color: #39cccc;
  background-size: cover;
  clip-path: polygon(0% 100%, 100% 100%, 0% 0%);
}

/* Center the demo */
html, body { height: 100%; }
body {
  display: flex;
  justify-content: center;
  align-items: center;
}

body {
  background-color: #333;
}