SOURCE

<div class="demo">
  hello world
</div>

/**
 * Folded corner effect — at an angle
 */

div {
  position: relative;
  width: 12em;
  background: #58a;
  /* Fallback */
  background: linear-gradient(-150deg, transparent 1.5em, #58a 0);
  padding: 2em;
  color: white;
  border-radius: .5em;
}

div::before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  background: linear-gradient(to left bottom, transparent 50%, rgba(0, 0, 0, .4) 0) 100% 0 no-repeat;
  width: 1.73em;
  height: 3em;
	transform:translateY(-1.3rem)  rotate(-30deg);
	transform-origin:bottom right;
	border-bottom-left-radius: .5em;
	box-shadow: -.2em .2em .3em -.1em rgba(0, 0, 0, .15)
}


/* 
div::before {
  content: '';
  position: absolute;
  top: 0;
  right: 0;
  width: 1.73em;
  height: 3em;
  background: linear-gradient(to left bottom, transparent 50%, rgba(0, 0, 0, .2) 0, rgba(0, 0, 0, .4)) 100% 0 no-repeat;
  transform: translateY(-1.3em) rotate(-30deg);
  transform-origin: bottom right;
  border-bottom-left-radius: .5em;
  box-shadow: -.2em .2em .3em -.1em rgba(0, 0, 0, .15)
} */