折角效果

div {
  width: 200px;
  aspect-ratio: 5 / 4;
  background: 
    linear-gradient(to left bottom, transparent 50%, rgba(0, 0, 0, .5) 0) no-repeat 100% 0 / 20px 20px,
    linear-gradient(-135deg, transparent 20px, #58a 0);
}