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CSS background-size

The CSS background-size property was introduced in CSS3 for the purposes of setting the dimensions of a background image.

The background-size property allows you to specify exactly what size your background image is. Furthermore, you can use the cover and contain keywords to scale the image to the box.

The background-size property is used in conjunction with the background-image property to define a background image on an HTML element.

In CSS3, the background-size property has been added to the background shorthand property. This means that you can set the background-size from within the background property (for example, background: url("background.png") 40% / 10em lightblue round fixed border-box;).

The examples on this page include browser-specific properties that start with extensions such as -webkit-, -moz-, etc. This is for browser compatibility reasons. See the bottom of this article for more on this.

Syntax: background-size: value

The formal syntax for this property is:

background-size: <bg-size> [ , <bg-size> ]*

Where

<bg-size> = [ <length> | <percentage> | auto ]{1,2} | cover | contain

For more information, see the Possible Values section below.
Example 
-webkit-background-size: contain; /* Safari and Chrome */
-moz-background-size: contain; /* Firefox */
-ms-background-size: contain; /* Internet Explorer */
-o-background-size: contain; /* Opera */
background-size: contain; /* CSS3 */
Note that this example includes various CSS extensions in addition to the W3C CSS3 property. This is for browser compatibility.
Try it yourself!


Possible Values:

Here are the possible values:

contain
Specifies that the background image should be scaled, while preserving its intrinsic aspect ratio (if any), to the largest size such that both its width and its height can fit inside the background positioning area.
cover
Specifies that the background image should be scaled, while preserving its intrinsic aspect ratio (if any), to the smallest size such that both its width and its height can fit inside the background positioning area.
<length>
Specifies the size explicitly using a lenth value. For example, 300px 100px will result in a background image being scaled to 300 pixels wide and 100 pixels high.

The first value provided defines the width of the background image. The second value provides the height. If only one value is given, the second becomes auto. For example, background-size:200px 50px provides a width of 200 pixels and a height of 50 pixels. A value of background-size:200px will result in the width being 200 pixels and the height being automatically determined by using the image's intrinsic ratio and the size of the other dimension, or failing that, using the image's intrinsic size, or failing that, treating it as 100%. In other words, the image should keep its proportions as it scales (i.e. not get distorted).

<percentage>
Specifies the size explicitly using a percentage value. A percentage value is relative to the background positioning area - not to the original image size. For example, 70% will result in a background image being scaled to 70 percent of the background positioning area.

As with the <length> values, the first value provides the width and the second provides the height. If the second value is missing, the height is automatically determined.

auto
Specifies that the image size is automatically determined using the intrinsic width and/or height of the image.
Initial Value: auto
Applies to: All elements
Inherited: No
Media: Visual
Animatable: Yes, except keyword values

Browser Compatibility

At the time of writing, CSS3 was still under development and browser support for many CSS3 properties was limited or non-existent. For maximum browser compatibility many web developers add browser-specific properties by using extensions such as -webkit- for Safari and Google Chrome, -ms- for Internet Explorer, -moz- for Firefox, -o- for 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.

Be aware that if you choose to use the proprietary CSS extensions in a live environment, your code will not pass any W3C CSS validation, as the browser-specific properties are not valid W3C properties.

Many of the CSS3 examples on this website include these browser specific properties. If they weren't included, most of the examples wouldn't work for most users (at least, not until possibly years after the article was written).

The major browser manufacturers are working to support the W3C properties, and eventually, you will be able to omit these browser-specific properties.

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