CSS3 “Lifted Corners” Drop-Shadow with Opacity

问题

I have been playing with some CSS3 drop-shadow effects. I am pretty fond of the "lifted corners" effect but I ran into an issue when attempting to add opacity to the element. My question is: Is there a way to create the "lifted corners" effect on an element with opacity?

http://jsfiddle.net/WAvZu/

.drop-shadow{
    position:relative;
    float:left;
    width:40%;
    padding:1em;
    margin:2em 10px 4em;
    background:#fff;
    -webkit-box-shadow:0 1px 4px rgba(0, 0, 0, 0.3), 0 0 40px rgba(0, 0, 0, 0.1) inset;
    -moz-box-shadow:0 1px 4px rgba(0, 0, 0, 0.3), 0 0 40px rgba(0, 0, 0, 0.1) inset;
    box-shadow:0 1px 4px rgba(0, 0, 0, 0.3), 0 0 40px rgba(0, 0, 0, 0.1) inset;
}
.drop-shadow:before,
.drop-shadow:after{
    content:"";
    position:absolute;
    z-index:-2;
}
.lifted{
    -moz-border-radius:4px;
    border-radius:4px;
}
.lifted:before,
.lifted:after{
    bottom:15px;
    left:10px;
    width:50%;
    height:20%;
    max-width:300px;
    max-height:100px;
    -webkit-box-shadow:0 15px 10px rgba(0, 0, 0, 0.7);
    -moz-box-shadow:0 15px 10px rgba(0, 0, 0, 0.7);
    box-shadow:0 15px 10px rgba(0, 0, 0, 0.7);
    -webkit-transform:rotate(-3deg);
    -moz-transform:rotate(-3deg);
    -ms-transform:rotate(-3deg);
    -o-transform:rotate(-3deg);
    transform:rotate(-3deg);
}
.lifted:after{
    right:10px;
    left:auto;
    -webkit-transform:rotate(3deg);
    -moz-transform:rotate(3deg);
    -ms-transform:rotate(3deg);
    -o-transform:rotate(3deg);
    transform:rotate(3deg);
}

回答1:

The problem is understanding stacking contexts and how they're rendered in the browser.

• the root element (HTML),
• positioned (absolutely or relatively) with a z-index value other than "auto",
• elements with an opacity value less than 1.
• on mobile WebKit and Chrome 22+, position: fixed always creates a new stacking context, even when z-index is "auto"

9.9.1 Specifying the stack level: the 'z-index' property

1. the background and borders of the element forming the stacking context.
2. the child stacking contexts with negative stack levels (most negative first).
3. the in-flow, non-inline-level, non-positioned descendants.
4. the non-positioned floats.
5. the in-flow, inline-level, non-positioned descendants, including inline tables and inline blocks.
6. the child stacking contexts with stack level 0 and the positioned descendants with stack level 0.
7. the child stacking contexts with positive stack levels (least positive first).

The background of #test is being rendered first since that is the element the opacity is being applied to. After that, the shadows go on top since they are in a new stacking context (position: absolute). And finally, the text of the div.

A simple solution: Would be to wrap the div in another div and apply the opacity to that div instead of the #test.

http://jsfiddle.net/WAvZu/3/

Another good read: What No One Told You About Z-Index

回答2:

After the discussion I'm not so sure about this to be honest, but I found this article: The stacking context.

As far as I know, the trick with z-index: -2 in your example just works because you did not set a z-index on .drop-shadow, which means it has no stacking context. Normally a child (:before and :after is some sort of child too) cannot have a lower z-index than it's parent, but it can be below if the parent has no stacking context.

The problem with opacity is, that it forms stacking context:

A stacking context is formed, anywhere in the document, by any element which is either

• the root element (HTML),
• positioned (absolutely or relatively) with a z-index value other than "auto",
• elements with an opacity value less than 1. (...),
• ...

With that said, you could work around by using a wrapper

<div class="wrapper">
    <div class="drop-shadow lifted">This is correct with opacity.</div>
</div>

and set the opacity there.

.wrapper {
    opacity: .5;
}

回答3:

I was able to fake a solution to this by adding a div inside of the container with the "lifted corners". It's a hack and I imagine that someone else could come up with a better solution but I thought I would post my findings in case anyone was curious.

http://jsfiddle.net/WAvZu/2/

来源：https://stackoverflow.com/questions/16067109/css3-lifted-corners-drop-shadow-with-opacity