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Mastering Box Shadows

Konstantin Kichinsky
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Today we will study the art of mastering shadows with CSS3. There are two types: box-shadow and text-shadow, defined respectively in the modules and CSS3 Text.

Both box and text work in a similar manner – the definition of text-shadow is written, pointing to the definition of box-shadow, with some exclusions. Let’s begin with box shadows:

Browser Support

CSS3 box-shadow is supported in IE10 and and other modern browsers. You can also use it in in your metro style apps for Windows 8 built with html/js.

box-shadow

First of all there is one thing you should remember: the shadow does not increase or decrease the size of the box or its border. The spec says: shadows do not trigger scrolling or increase the size of the scrollable area.

To create the most basic shadow, all you need is to define the first two parameters of the rule: horizontal and vertical offset of the shadow (1.1):

box-shadow: 3px 3px;

A positive value moves the shadow to the right and down, and a negative one to the left and up.

If you don’t specify the shadow’s color, most browsers will use the text color in current context (1.2). (The spec changed in February 2012, in previous edition omitted box-shadow “colors were UA-chosen colors”. So you may find some browsers with a different behavior.):

box-shadow: 3px 3px; color:blue;

To set the color of the shadow, you need to add a color value at the end (1.3):

box-shadow: 3px 3px darkgreen;

To define the color you can use any of the available CSS3 notations: #RGB, #RRGGBB, namely, by rgb() or rgba() and hsla() functions with alpha-channel. The hsla-function is very useful while building complex samples.

Blurring

The third one length you can set is a blur radius — a positive value defining how much the shadow’s edge is blurred (2.1–2.3):

box-shadow:3px 3px 3px darkgrey;

By default, the blur-radius equals zero and the shadow’s edge is sharp.

By combining blurring with the shadow’s offsets, you can achieve many effects. On the sample (2.3) both offsets are equal to zero, but thanks to the blurring effect there’s a shadow around the box:

box-shadow:0 0 9px black;

The blurring algorithm is not defined by the spec itself, but it says that the blurring effect should approximate the Gaussian blur with a standard deviation equal to half of the blur radius. In other words it means that the actual shadow generated by different UAs can differ at times.

The fourth length is for spread-distance, this expands or contracts the shadow. By default, the shadow’s size equals to the box’s, but by using the spread you can change it. Also, I should note here that this parameter was introduced in later spec versions, so there’s many out of date tutorials on the wild web with no idea on its existence. ;)

To expand the shadow, set a positive spread-distance (3.1, 3.2):

box-shadow:6px 6px 0px 4px darkred;

To contract — use a negative one (3.3):

box-shadow:12px 12px 8px -4px darkred;

On the sample above (3.4) the shadow has horizontal and vertical offsets of 6px to the left and down, and is expanded for 8px from each side:

box-shadow:6px 6px 0 8px grey;

If you use rounded corners on your box, expect that the border-radius of the expanded shadow will be also scaled up (3.5):

Inner Shadow

One more interesting modifier you can define is the “inset” keyword, which allows you to draw an inner shadow in your box (4.1-4.4):

box-shadow:inset 4px 4px rgba(66,66,66,0.5); /* (4.1) */
box-shadow:inset 4px 4px 0 8px rgba(198,198,198,1); /* (4.2) */
box-shadow:inset -2px -2px 8px 0px black; /* (4.3) */
box-shadow:inset 0 0 4px 0px black; /* (4.4) */

Note that the inner shadow is drawn only inside the box, and if you use positive spread-distance on inner shadow it means contracting the shadow’s perimeter shape (4.2).

Multiple Shadows

And the last one piece of the mosaic: you can define as much shadows for the same box as you want by writing all of them comma-separated in the same rule.

To create a rainbow shadow (5.1) all you need to do is to write up seve shadows with increasing spread-distance:

box-shadow: 0 0 2px 1px red,

           0 0 2px 2px orange,

           0 0 2px 3px yellow,

           0 0 2px 4px green,

           0 0 2px 5px lightblue,

           0 0 2px 6px blue,

           0 0 2px 7px violet;

Note that the shadow effects are applied front-to-back. The violet shadow one will be drawn first, on top of it will be drawn the blue one and so on. The red one will be drawn at the end, on top of other shadows, and finally — the box itself with all its content.

As all the shadows are independent, you can easily combine shadows with different offsets (5.2):

box-shadow: -6px -6px 8px -4px rgba(255,0,0,0.75),

           6px -6px 8px -4px rgba(0,255,0,0.75),

           6px 6px 8px -4px rgba(255,255,0,0.75),

           -6px 6px 8px -4px rgba(0,0,255,0.75);

Or combine outer and inner shadows (5.3):

box-shadow: inset 0 0 8px lightgray,

           1px 1px 3px darkgray;

Here is an underline (bottom shadow) sample (5.4):

box-shadow: 0 1px red,

           0 3px 3px -2px black

By adding some special effects on the :before and :after pseudo-classes with content, you can create a slick-box described by Matt Hamm (5.5):

.slick-box {

   position: relative;

   height: 50px;

   border: 1px solid #efefef;

   background: #fff;

   box-shadow: 0 1px 4px rgba(0, 0, 0, 0.27), 0 0 40px rgba(0, 0, 0, 0.06) inset;

}

 

.slick-box:before, .slick-box:after {

   content: '';

   z-index: -1;

   position: absolute;

   left: 10px;

   bottom: 10px;

   width: 70%;

   max-width: 300px; /* avoid rotation causing ugly appearance at large container widths */

   height: 55%;

   box-shadow: 0 8px 16px rgba(0, 0, 0, 0.3);

   transform: skew(-15deg) rotate(-6deg);                

}

           

.slick-box:after {

   left: auto;

   right: 10px;

   transform: skew(15deg) rotate(6deg);                

}

To simplify the code I’ve removed all vendor prefixes, but while trying to reproduce it you should use all of these, -ms-transform, -webkit-transform and so on.

Common syntax

Summing up the common syntax for box-shadow looks like this:

box-shadow:  [ ,  ]*;

 = inset? && [ {2,4} && ? ]

The last one rule in a full version means:

box-shadow: inset? h-offset v-offset blur-radius spread-distance color;

The blur-radius and the spread-distance can be omitted. The inset keyword switched the shadow form outer to inner.

Interactive Sample

If you wish to play with shadows in an interactive way my colleagues created a cool demo for the Build conference in the last September: “Hands-on: box-shadow“.

A Note

The CSS properties discussed in this article are defined in the CSS3 Backgrounds and Borders module, which is currently in the Working Draft status. Meanwhile, it seems to be quite stable though it still can change in details.

CSS Master, 3rd Edition