SOURCE

<div class="fold-1">折角效果哦</div>

.fold-1 {
  margin-left: auto;
  margin-right: auto;
  margin-bottom: 20px;
  position: relative;
  width: 200px;
  height: 80px;
  border-radius: .5em;
  color: #fff;
  line-height: 80px;
  text-align: center;
  background: linear-gradient(-150deg, transparent 1.5em, #58a 0);
}

.fold-1::before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  background: linear-gradient(to left bottom, transparent 50%, rgba(0, 0, 0, .2) 0, rgba(0, 0, 0, .4)) 100% 0 no-repeat;
  width: 1.73em;
  height: 3em;
  transform: translateY(-1.3em) rotate(-30deg);
  transform-origin: bottom right;
  border-bottom-left-radius: inherit;
  box-shadow: -.2em .2em .3em -.1em rgba(0, 0, 0, .15);
}