Box Shadow inset not working

问题

So I've been using a shadow box inset to make a inner glow kind of making the edges blurry and shadowy like for a edge burn look. I'm trying to use it for the top and bottom only and not for the left/right sides. But it's not working. I'm using it on a overflow: auto <div> so that it can scroll and have a nice effect.

Here's my css:

#content {
    font: 14px "Lucida Grande", "Lucida Sans Unicode", sans-serif; 
    line-height:1.2em;
    height: 400px;
    width: 500px;
    overflow: auto;
    float: right;
    padding: 0 10px;

    -moz-box-shadow: inset 0  8px 8px -8px #000, inset 0 -8px 8px -8px #000;
    -webkit-box-shadow: inset 0  8px 8px -8px #000, inset 0 -8px 8px -8px #000;
    box-shadow: inset 0  8px 8px -8px #000, inset 0 -8px 8px -8px #000;
}

回答1:

You have a black shadow on a black background so naturally, you're not going to see anything. Turning off your black background, we can see the shadows just fine...

http://jsfiddle.net/sparky672/p3Mgn/1/show

So you just need to select different shadow colors. Here are your shadows changed to white #fff...

Full Size Demo

http://jsfiddle.net/sparky672/p3Mgn/3/

-moz-box-shadow: inset 0  8px 8px -8px #fff, inset 0 -8px 8px -8px #fff;
-webkit-box-shadow: inset 0  8px 8px -8px #fff, inset 0 -8px 8px -8px #fff;
box-shadow: inset 0  8px 8px -8px #fff, inset 0 -8px 8px -8px #fff;

EDIT in response to OP's comments:

The browser is only given two colors to use in order to render a shadow.

1) The background image's color (or just background color in this case)

2) The shadow color

Wherever they're both the same, the shadow will be invisible.

To have a blurry effect using a black background, perhaps try #444 for the shadow... it looks pretty good I think...

http://jsfiddle.net/sparky672/p3Mgn/5/show/

回答2:

Do you want to have shadow above content to blur top and bottom? If yes then the problem is that you shadow is shown below content. You can make it above it if you set "position: relative; z-index: -1;" to content block, but then you will not be able to click or scroll it.

Easier way to achive this effect is to use :before and :after pseudoclasses and css-gradients.

Example here: http://jsfiddle.net/V96wx/2/

In my example above you will need 2 containers — one for overflow and one for fades (to make it more bulletproof). But theoretically you can do it with only 1 container, I'll write how...

First of all — how :before and :after works. Simplest way to think about them is as about 2 more elements that will be added inside parent container before and after actual content. For example: .about:before will be added inside .about container, but before actual content.

:before and :after have one required property content if you didn't add it, element will not be created. conent may have one of the following values: htmldog.com/reference/cssproperties/content. In my example it was left blank. After element is inserted you can style it as you wish by the same rules you style every other element.

To make fade in my examples I used gradient with trasparency. You can read about gradients here davidwalsh.name/css-gradients. Transparecy is done by using colors in rgba (4th digit is transparency level).

The reason why I used 2 containers in my example is because it is harder to accurately position :before and :after elements above main container without it — if you try to use realtive coordinates for them they will position rightly, but will scroll with content and if you not use position: relative on base container you will need to know this container coordinates to make positioning. It is not a problem if container height is fixed but may be tricky if it is not.

BTW: Theoretically there is an even easier way to do fade — by using css masks with gradients ( webkit.org/blog/181/css-masks ) but right now it's working only in Safari and Chrome.

来源：https://stackoverflow.com/questions/7856528/box-shadow-inset-not-working

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