CSS opacity

The CSS opacity property was introduced in CSS3 to enable developers to enable transparency in HTML elements.

The opacity property allows you to specify a level of opacity against an object so that it becomes semi-transparent, or even fully transparent (if that's the desired effect).

Syntax

opacity: value

Possible Values

Here are the possible values:

<alphavalue>
Specifies the amount of transparency that should be applied to the element. A value of 0.0 specifies fully transparent and a value of 1.0 specifies fully opaque (no transparency).

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
1
Applies To
All elements
Inherited?
No
Media
Visual
Animatable
Yes (see example)

Example Code

Basic CSS

opacity: 0.4;

Working Example within an HTML Document

<!doctype html>
<title>Example</title>
<style>
div {
  height: 100px;
  width: 200px;
  border: 1px solid black;
}
.bottom { 
  background-color: gold;
  z-index: 1;
}
.top {
  position: relative;
  top: -65px;
  left: 50px;
  background-color: lime;
  z-index: 2;
  opacity: 0.4;
}
</style>
<div class="bottom"></div>
<div class="top"></div>

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CSS Specifications

More Transparency

The opacity property isn't the only way to specify opacity/transparency in CSS. CSS3 provides for alpha colors, as well as the transparent keyword. More about these below.

Alpha Color Functions

CSS3 has introduced rgba() and hsla() color functions, which allow you to specify the opacity at the color level.

For example, background-color: rgba(0,0,0,0.4) provides the color black (i.e. 0,0,0) with an opacity of 0.4. And for an HSLA example, background-color: hsla(0, 100%, 50%, 0.8) specifies the color red (i.e. 0, 100%, 50%) with an opacity of 0.8.

The transparent Keyword

The transparent keyword can be applied against any element when using the background-color, border-color or color properties.

The CSS transparent keyword specifies that the element should be fully transparent. It is shorthand for rgba(0,0,0,0) (transparent black).

In CSS 1, the transparent keyword was limited to the background-color property. In CSS level 2, it was extended to the border-color property. In CSS 3, the transparent keyword has been extended to any element that uses the color property.

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.