Simple CSS-only corner sash/ribbon. Works down to IE8.

demo.html

<div class="box">
  <p>The <kbd>.stash</kbd> element creates the bounding box that aligns in the corner and crops the contents at an angle. The <kbd>.sash > span</kbd> gets sized and rotated.</p>
  <span class="sash">
    <span>New!</span>
  </span>
</div>

sash.scss

@mixin _rotate($deg: -45, $display: block) {
  $matrix: (cos($deg) -1*sin($deg) sin($deg) cos($deg));
  display: $display;
  @include rotate(#{$deg}deg);
  @include transform-origin(center, bottom, 0, false);

  .no-csstransforms & {
    // http://stackoverflow.com/questions/5669489/create-a-cross-browser-mixin-for-transform-rotate
    // http://www.useragentman.com/IETransformsTranslator/
    -ms-filter: "progid:DXImageTransform.Microsoft.Matrix(M11=#{nth($matrix, 1)}, M12=#{nth($matrix, 2)}, M21=#{nth($matrix, 3)}, M22=#{nth($matrix, 4)}, SizingMethod='auto expand')";
    margin: 0 0 0 0;
  }
}

.sash {
  $padding: baseline(1);
  $height: (1.5 * $em-base) + ($padding * 2);
  $hypot: 1.42;
  $bgColor: #8BBDF7;
  $bgShade: shade($bgColor, 12%);
  $overhang: -1px;
  $width: 6.5em;

  position: absolute;
  z-index: 50;
  top: $overhang;
  left: $overhang;
  width: $width;
  height: $width;
  overflow: hidden;

  span {
    @include _rotate(-45deg);
    @include background(linear-gradient(left, $bgShade, $bgColor, $bgShade));
    @include rem(font-size, 0.9rem);
    @include box-shadow(0 1px 4px 0 rgba($black, 0.5));
    position: absolute;
    background: $bgColor;
    padding: $padding;
    text-align: center;
    font-weight: $font-weight-bold;
    line-height: 1.5;
    color: $white;
    display: block;
    width: 100% * $hypot;
    bottom: 51%;
    left: -21%;

  }
  .no-csstransforms & {
    span {
      left: -24%;
      bottom: 90%;
      border-bottom: 1px solid #888;
    }
  }
}

// Demo css
.box {
  background: #ddd;
  position: relative;
  padding: 1em;
  margin: 1em;
}

Not fully tested in IE yet, but the arbitrary rotation does work in IE8. Just need to adjust the math.

Still to test:

• IE9-11