See http://blog.caurea.org/2011/02/27/pure-css-page-curl-effect.html

_curl-effect.scss

/* Stupid compass box-shadow mixin doesn't support multiple shadows... */
@mixin box-shadow-2($first, $second) {
  $full: $first, $second;
  @include experimental(box-shadow, $full, -moz, -webkit, -o, not -ms, not -khtml, official);
}

@mixin curl-effect-shadow($corner) {
  z-index: -1;
  position: absolute;
  background: transparent;
  width: 40%;
  height: 40%;
  content: '';
  bottom: 0;
  #{$corner}: 0;

  $mul: +1;
  @if $corner == left { $mul: -1; }
  @include apply-origin($corner bottom);
  @include transform(1, $mul * 10deg, 0, 0, $mul * 20deg);
  @include box-shadow(rgba(0, 0, 0, 0.3), 0, 8px, 16px);
}

@mixin curl-effect($background: #ffffff) {
  position: relative;

  background: $background;
  border: 1px solid lighten($background, 10%);

  @include box-shadow-2(
    0 1px 4px rgba(0, 0, 0, 0.27),
    0 0 60px rgba(0, 0, 0, 0.10) inset
  );

  &:before {
    @include curl-effect-shadow(left);
  }

  &:after {
    @include curl-effect-shadow(right);
  }
}