box-shadow

Used in casting shadows (often called “Drop Shadows”, like in Photoshop) from elements. Here is an example with the deepest possible browser support:

.shadow {
  -webkit-box-shadow: 3px 3px 5px 6px #ccc;  /* Safari 3-4, iOS 4.0.2 - 4.2, Android 2.3+ */
  -moz-box-shadow:    3px 3px 5px 6px #ccc;  /* Firefox 3.5 - 3.6 */
  box-shadow:         3px 3px 5px 6px #ccc;  /* Opera 10.5, IE 9, Firefox 4+, Chrome 6+, iOS 5 */
}

Thats:

box-shadow: [horizontal offset] [vertical offset] [blur radius] [optional spread radius] [color];

1. The horizontal offset (required) of the shadow, positive means the shadow will be on the right of the box, a negative offset will put the shadow on the left of the box.
2. The vertical offset (required) of the shadow, a negative one means the box-shadow will be above the box, a positive one means the shadow will be below the box.
3. The blur radius (required), if set to 0 the shadow will be sharp, the higher the number, the more blurred it will be, and the further out the shadow will extend. For instance a shadow with 5px of horizontal offset that also has a 5px blur radius will be 10px of total shadow.
4. The spread radius (optional), positive values increase the size of the shadow, negative values decrease the size. Default is 0 (the shadow is same size as blur).
5. Color (required) – takes any color value, like hex, named, rgba or hsla. If the color value is omitted, box shadows are drawn in the foreground color (text color). But be aware, older WebKit browsers (pre Chrome 20 and Safari 6) ignore the rule when color is omitted.

Using a semi-transparent color like rgba(0, 0, 0, 0.4) is most common, and a nice effect, as it doesn’t completely/opaquely cover what it’s over, but allows what’s underneath to show through a bit, like a real shadow.

Example

Inner Shadow

.shadow {
   -moz-box-shadow:    inset 0 0 10px #000000;
   -webkit-box-shadow: inset 0 0 10px #000000;
   box-shadow:         inset 0 0 10px #000000;
}

Example

Internet Explorer (8 and down) Box Shadow

You need an extra element, but it’s do-able with filter.

<div class="shadow1">
  <div class="content">
    Box-shadowed element
  </div>
</div>

.shadow1 {
  margin: 40px;
  background-color: rgb(68,68,68); /* Needed for IEs */

  -moz-box-shadow: 5px 5px 5px rgba(68,68,68,0.6);
  -webkit-box-shadow: 5px 5px 5px rgba(68,68,68,0.6);
  box-shadow: 5px 5px 5px rgba(68,68,68,0.6);

  filter: progid:DXImageTransform.Microsoft.Blur(PixelRadius=3,MakeShadow=true,ShadowOpacity=0.30);
  -ms-filter: "progid:DXImageTransform.Microsoft.Blur(PixelRadius=3,MakeShadow=true,ShadowOpacity=0.30)";
  zoom: 1;
}
.shadow1 .content {
  position: relative; /* This protects the inner element from being blurred */
  padding: 100px;
  background-color: #DDD;
}

One-Side Only

Using a negative spread radius, you can get squeeze in a box shadow and only push it off one edge of a box.

.one-edge-shadow {
  -webkit-box-shadow: 0 8px 6px -6px black;
     -moz-box-shadow: 0 8px 6px -6px black;
          box-shadow: 0 8px 6px -6px black;
}

Multiple Borders & More

You can comma separate box-shadow any many times as you like. For instance, this shows two shadows with different positions and different colors on the same element:

box-shadow: 
  inset 5px 5px 10px #000000, 
  inset -5px -5px 10px blue;

Browser Support

See snippet at top of page for specifics on vendor prefix coverage. This is one of those properties where dropping the prefixes is pretty reasonable at this point.