CSS background-image-opacity?

If the background doesn't have to repeat, you can use the sprite technique (sliding-doors) where you put all the images with differing opacity into one (next to each other) and then just shift them around with background-position.

Or you could declare the same partially transparent background image more than once, if your target browser supports multiple backgrounds (Firefox 3.6+, Safari 1.0+, Chrome 1.3+, Opera 10.5+, Internet Explorer 9+). The opacity of those multiple images should add up, the more backgrounds you define.

This process of combining transparencies is called Alpha Blending and calculated as (thanks @IainFraser):

αᵣ = α₁ + α₂(1-α₁) where α ranges between 0 and 1.

.class {
    /* Fallback for web browsers that doesn't support RGBa */
    background: rgb(0, 0, 0);
    /* RGBa with 0.6 opacity */
    background: rgba(0, 0, 0, 0.6);
}

Copied from: http://robertnyman.com/2010/01/11/css-background-transparency-without-affecting-child-elements-through-rgba-and-filters/

You can do the faded background with CSS Generated Content

Demo at http://jsfiddle.net/gaby/WktFm/508/

Html

<div class="container">
        contents
</div>

Css

.container{
    position: relative;
    z-index:1;
    overflow:hidden; /*if you want to crop the image*/
}
.container:before{
    z-index:-1;
    position:absolute;
    left:0;
    top:0;
    content: url('path/to/image.ext');
    opacity:0.4;
}

But you cannot modify the opacity as we are not allowed to modify generated content..

You could manipulate it with classes and css events though (but not sure if it fits your needs)

for example

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

UPDATE

You can use css transitions to animate the opacity (again through classes)

demo at http://jsfiddle.net/gaby/WktFm/507/

Adding

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

to the .container:before rule will make the opacity animate to 1 in one second.

Compatibility

• FF 5 (maybe 4 also, but do not have it installed.)
• IE 9 Fails..
• Webkit based browsers fail (Chrome supports it now v26 - maybe earlier versions too, but just checked with my current build), but they are aware and working on it ( https://bugs.webkit.org/show_bug.cgi?id=23209 )

.. so only the latest FF supports it for the moment.. but a nice idea, no ? :)