folded-corner

folded-corner.scss

@mixin folded-corner($background, $size, $angle, 30deg) {
  position: relative;
  background: linear-gradient($angle - 180deg, transparent $size, $background, 0);
  border-radius: .5em;
  
  $x: $size / sin($angle);
  $y: $size / cos($angle);

  
  &::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: $y;
    height: $x;
    transform: translateY($y - $x) rotate(2 * $angle - 90deg);
    transform-origin: bottom right;
    border-bottom-left-radius: inherit;
    box-shadow: -.2em .2em .3em -.1em rgba(0, 0, 0, .2);
  }
}

sin / cos use mathsass: https://github.com/terkel/mathsass