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
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|---|---|
| Example |
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| Try it yourself! |
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| Possible Values: |
Here are the possible values:
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| Initial Value: | 1
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| Applies to: | All elements |
| Inherited: | No |
| Media: | Visual |
More Transparency Please!
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 Colors
CSS3 has introduced RGBA and HSLA color values, 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(120, 100%, 50%, 0.8) specifies the color green (i.e. 120% 100%) with an opacity of 0.8.
CSS transparent Keyword
The transparent keyword can be applied against any element when using the background-color, border-color or color properties.
The 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 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.