How Can I Control Background Image Opacity with CSS Generated Content?

CSS Background Image Opacity Made Possible with Generated Content

In contrast to the opacity adjustment of background colors, adjusting the alpha value of background images has been previously deemed challenging. However, CSS Generated Content offers a solution for dynamically altering the opacity of background images.

To implement this technique, create a container div with specified position and dimensions, and populate it with the remaining content elements. Next, define the background image within the pseudo-element created for the container. The opacity property can then be manipulated within the pseudo-element.

An example demonstration can be found at http://jsfiddle.net/gaby/WktFm/508/. Below is the necessary code snippet:

HTML

<div>
<p><strong>CSS</strong></p>
<pre class="brush:php;toolbar:false">.container {
  position: relative;
  z-index: 1;
  overflow: hidden;
}

.container:before {
  z-index: -1;
  position: absolute;
  left: 0;
  top: 0;
  content: url('path/to/image.ext');
  opacity: 0.4;
}

While the opacity of generated content cannot be modified directly, it can still be dynamically controlled through CSS events and classes. For instance, the following code would adjust the opacity to 1 upon hover:

.container:hover:before {
  opacity: 1;
}

Additionally, CSS transitions can be employed to animate opacity changes smoothly. By adding the following properties to the .container:before rule, the opacity will transition to 1 over 1 second:

-webkit-transition: opacity 1s linear;
-o-transition: opacity 1s linear;
-moz-transition: opacity 1s linear;
transition: opacity 1s linear;

It's important to note the varying compatibility of this technique across different browsers. Firefox 5 and later support it, but Internet Explorer 9 and below do not. Webkit-based browsers like Chrome may have inconsistent support based on their version.