CSS box-shadow

The box-shadow property is one of the properties introduced in CSS3 to enable developers with the ability to add shadow effects to HTML elements.

The CSS box-shadow property is used for attaching one or more drop shadows to an HTML element. You can create drop shadows on any HTML element. You can even give them that "blurred" effect (also known as "Gaussian blur").

You can specify which color the drop shadow should be as well as it's offset from its box (i.e. how far it should drop down/away from the HTML element that you apply the drop shadow to).

The box-shadow property accepts a comma-separated list of shadows, each specified by 2-4 length values, an optional color, and an optional inset keyword. Omitted lengths are 0; omitted colors are chosen by the browser/user agent.

Syntax

box-shadow: <horizontal_offset> <vertical_offset> <blur> <spread_distance> <color>

The formal grammar for this property is:

box-shadow: none | <shadow> [ , <shadow> ]*

Where

<shadow> = inset? && [ <length>{2,4} && <color>? ]

See below for an explanation.

Possible Values

To recap, the syntax is:

box-shadow: <horizontal_offset> <vertical_offset> <blur> <spread_distance> <color>

Here's an explanation of these values:

none
This keyword specifies that no drop shadow will be added to the element. If you use this keyword, there's no need to add the other values.
<horizontal_offset>
The first value determines the horizontal offset of the drop shadow. It is a length value. For example, 10px or 1em. Negative values are allowed. If a negative value is provided, the offset will result in the drop shadow being drawn to the left of the box. If it is a positive value, it will be drawn to the right.
<vertical_offset>
The second value determines the vertical offset of the drop shadow. It is a length value. For example, 10px or 1em. Negative values are allowed. If a negative value is provided, the offset will result in the drop shadow being drawn above the box. If it is a positive value, it will be drawn below.
<blur>
The third value determines the Gaussian blur effect (if any). If the value is zero (i.e. 0), the edge of the shadow will be sharp. The higher the value, the more blurred the shadow will become. Negative values are not allowed for a Gaussian blur.
<spread_distance>
The fourth value determines the size of the drop shadow. Adding a positive value here will increase the size of the drop shadow - it will expand in all directions. A negative value will decrease the size (it will contract).
<color>
The fifth value determines the color of the drop shadow. For example, steelblue, #FF4500 or rgba(0,0,0,0.3) are all valid colors. If this value isn't provided, the value will be taken from the color property.
inset
This (optional) keyword specifies that the drop shadow will be added to the inside of the element, making it an "inner shadow". This can give the element the appearance being "cut out" of the canvas/page (as opposed to an "outer shadow" in which the element appears to be hovering above the page). If the inset keyword is omitted, the drop shadow will be an "outer shadow".

In addition, all CSS properties also accept the following CSS-wide keyword values as the sole component of their property value:

initial
Represents the value specified as the property's initial value.
inherit
Represents the computed value of the property on the element's parent.
unset
This value acts as either inherit or initial, depending on whether the property is inherited or not. In other words, it sets all properties to their parent value if they are inheritable or to their initial value if not inheritable.

Basic Property Information

Initial Value
none
Applies To
All elements
Inherited?
No
Media
Visual
Animatable
Yes (see example)

Example Code

Basic CSS

box-shadow: 3px 3px 10px 0px rgba(0,0,0,0.3);

Working Example within an HTML Document

<!doctype html>
<title>Example</title>
<style>
div {
  width: 200px;
  padding: 30px;
  border: 1px solid limegreen;
  margin: 30px;
}
.drop-shadow {
  box-shadow: 7px 7px 20px 0px  rgba(0,0,0,0.3), 40px 40px 10px -40px  rgba(0,0,0,0.8) inset;
}
</style>
<div class="drop-shadow">
  This box has a drop shadow and an inset shadow
</div>

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CSS Specifications

Browser Support

The following table provided by Caniuse.com shows the level of browser support for this feature.

Vendor Prefixes

For maximum browser compatibility many web developers add browser-specific properties by using extensions such as -webkit- for Safari, Google Chrome, and Opera (newer versions), -ms- for Internet Explorer, -moz- for Firefox, -o- for older versions of Opera etc. As with any CSS property, if a browser doesn't support a proprietary extension, it will simply ignore it.

This practice is not recommended by the W3C, however in many cases, the only way you can test a property is to include the CSS extension that is compatible with your browser.

The major browser manufacturers generally strive to adhere to the W3C specifications, and when they support a non-prefixed property, they typically remove the prefixed version. Also, W3C advises vendors to remove their prefixes for properties that reach Candidate Recommendation status.

Many developers use Autoprefixer, which is a postprocessor for CSS. Autoprefixer automatically adds vendor prefixes to your CSS so that you don't need to. It also removes old, unnecessary prefixes from your CSS.

You can also use Autoprefixer with preprocessors such as Less and Sass.