Writing: "box-shadow: h-shadow v-shadow blur spread color inset"; h-shadow is the horizontal position, v-shado is the vertical position, spread is the shadow size, and inset changes the outer shadow to Inner shadow (can be omitted).
The operating environment of this tutorial: Windows 7 system, CSS3&&HTML5 version, Dell G3 computer.
In CSS, you can use the box-shadow attribute to achieve the border shadow effect. The box-shadow attribute can set one or more drop-down shadow boxes.
Syntax
box-shadow: h-shadow v-shadow blur spread color inset;
Possible values:
| Values | Description |
|---|---|
| h-shadow | Required. The position of the horizontal shadow. Negative values allowed |
| v-shadow | Required. The position of the vertical shadow. Negative values allowed |
| blur | Optional. Fuzzy distance |
| #spread | Optional. The size of the shadow |
| #color | is optional. The color of the shadow. |
| inset | Optional. Change shadow to inner shadow from outer shadow (at start) |
注意：boxShadow 属性把一个或多个下拉阴影添加到框上。该属性是一个用逗号分隔阴影的列表，每个阴影由 2-4 个长度值、一个可选的颜色值和一个可选的 inset 关键字来规定。省略长度的值是 0。
box-shadow属性的使用
1、水平垂直偏移为0也可以有阴影
如果offset-x或offset-y值为0，则阴影在元素背后，此时给blur-radius值或spread值可以产生阴影效果。
例子：
第一个div通过设置blur-radius产生阴影效果。
第二个div通过设置spread正值产生阴影效果。
第三个div通过设置spread负值产生阴影效果。
但是有一点要注意：扩展阴影必须和阴影模糊半径配合使用。
我个人觉得应该是没有配合使用这一说，但不可能只设置扩展阴影，因为扩展阴影和阴影模糊的取值都可以为正。如果只有扩展阴影的话，会被浏览器当做模糊阴影来解析，所以也可以简单理解为“扩展阴影必须和阴影模糊半径配合使用”，如果只用扩展阴影，可以写成：box-shadow:0 0 0 1px;。
<style type="text/css">
div{
width: 100px;
height: 100px;
margin:50px;
border: 10px dotted pink;
display: inline-block;
}
.blur{
box-shadow: 0 0 20px ;
/*box-shadow: 0 0 20px green;*/ /*也可以自定义颜色*/
}
.spread-positive{
box-shadow: 0 0 20px 5px ;
/* box-shadow: 0 0 20px 5px green;*/ /*也可以自定义颜色*/
}
.spread-negative{
box-shadow: 0 0 20px -5px ;
/* box-shadow: 0 0 20px -5px green;*/ /*也可以自定义颜色*/
}
</style>
<body>
<div class="blur"></div>
<div class="spread-positive"></div>
<div class="spread-negative"></div>
</body>
2、设置水平垂直偏移得到阴影效果
outset情况：水平垂直偏移为0，但是不设置blur和spread，看不到阴影，因为此时box-shadow的周长和border-box一样，所以可以通过设置偏移让阴影显示出来。
inset情况：水平垂直偏移为0，不设置blur和spread，同样看不到阴影，因为此时box-shadow的周长和padding-box一样，同样可通过设置偏移让阴影显示出来。
例子:
<style type="text/css">
div{
width: 100px;
height: 100px;
margin:50px;
border: 10px dotted pink;
display: inline-block;
}
.shadow0{box-shadow: 0 0;}
.shadow1{box-shadow: 1px 1px;}
.shadow10{box-shadow: 10px 10px;}
.inset-shadow0{box-shadow: 0 0 inset;}
.inset-shadow1{box-shadow: 1px 1px inset;}
.inset-shadow10{box-shadow: 10px 10px inset;}
</style>
<body>
<div class="shadow0"></div>
<div class="shadow1"></div>
<div class="shadow10"></div>
<div class="inset-shadow0"></div>
<div class="inset-shadow1"></div>
<div class="inset-shadow10"></div>
</body>
3、投影方式
投影方式默认是outset，即外部投影，可设置inset让向内投影。
例子：第一个div默认outset，第二个设置inset，第三个同时设置两个阴影可以更好的看到outset和inset的关系，第四个div可以看出inset阴影在背景之上，内容之下。
<style type="text/css">
div{
width: 100px;
height: 100px;
margin:50px;
border: 10px dotted pink;
display: inline-block;
vertical-align: top;
}
.outset{
box-shadow: 10px 10px teal;
}
.inset{
box-shadow: 10px 10px teal inset;
}
.double{
box-shadow: 10px 10px teal inset,10px 10px teal;
}
.bg{
background-color: yellow;
}
</style>
<body>
<div class="outset"></div>
<div class="inset"></div>
<div class="double"></div>
<div class="inset bg">inset阴影在背景之上，内容之下</div>
</body>
4、如果元素同时指定border-radius属性，则阴影呈现相同的圆角。
<style type="text/css">
div{
width: 100px;
height: 100px;
margin:50px;
border: 10px dotted pink;
display: inline-block;
border-radius: 50px;
}
.shadow{
box-shadow: 0 0 10px 10px green;
}
</style>
<body>
<div class="shadow"></div>
</body>
5、经典例子
w3c中的一个例子。http://www.w3.org/TR/css3-background/#the-box-shadow
可见：
- border-radius会以相同的作用影响阴影外形
- border-image,padding不会影响阴影的任何外形
- 阴影box和box模型一样
- 外阴影在对象背景之下，内阴影在背景之上。
- 层次：内容>内阴影>背景图片>背景颜色>外阴影
6、多重阴影
这个效果在上面就看到了，现在再补充一些内容。
语法：可以设置任意多个阴影，用逗号隔开。
一个box有多重阴影时，需要注意顺序：多个阴影从上往下分布，第一个阴影在最顶层。
举例：单边阴影效果
先解释一下：可单独设置左边框的阴影，右边框的阴影，上边框的阴影，下边框的阴影，其实这样说也对，因为效果看起来就是这样，但根本原因是阴影在盒子后面，只是让阴影的位置发生了变化，其他3 个边的阴影还是存在的，只是被覆盖住了而已，所以，设置某个边的阴影是个很虚的东东了，哎，网上这种说法初看还让我略感困惑，所以我这里说是单边阴影效果，告诉大家只是一种效果，本质还是个box。
例子解释：给第一个div的上右下左border分别设置红橙黄绿，四种颜色，则red-shadow在最顶层，green-shadow在最底层，如下图左。
给其加上blur模糊半径，效果更明显，如下图中，可见red-shadow的模糊半径不受干扰，因为在最顶层；接下来orange-shadow次之，被red-shadow的radius干扰；yellow-shadow被orange-shadow和red-shadow的radius干扰；同理green-shadow被它上面的所有shadow的radius干扰。
如果还是不太理解，那给red-shadow设置一个很大的radius，比如50，就可以看到非常明显的效果了，见下图右。
<style type="text/css">
div{
width: 100px;
height: 100px;
margin:50px;
display: inline-block;
border: 10px dotted pink;
}
.shadow{
box-shadow: 0 -5px red,
5px 0 orange,
0 5px yellow,
-5px 0 green;
}
.blur-shadow{
box-shadow: 0 -5px 5px red,
5px 0 5px orange,
0 5px 5px yellow,
-5px 0 5px green;
}
.big-redShadow{
box-shadow: 0 -5px 50px red,
5px 0 5px orange,
0 5px 5px yellow,
-5px 0 5px green;
}
</style>
<body>
<div class="shadow"></div>
<div class="blur-shadow"></div>
<div class="big-redShadow"></div>
</body>
7、阴影和布局
阴影不影响布局， 但是可能会覆盖其他box或者其他box的阴影。
阴影不触发滚动条，也不增加滚动区域的大小。
所以布局时可忽略阴影。
8、spread妙用
用spread模拟实现border
<style type="text/css">
div{
width: 100px;
height: 100px;
display: inline-block;
margin:10px;
vertical-align: top;
}
.border{
border:1px solid red;
}
.spread{
box-shadow: 0 0 0 1px red;
}
.muli-border{
box-shadow: 0 0 0 2px red,0 0 0 4px green,0 0 0 6px blue;
}
</style>
<body>
<div class="border">border</div>
<div class="spread">box-shadow</div>
<div class="muli-border">多重<br/>box-shadow</div>
</body>
用spread实现双色方括号
<style type="text/css">
.decorator {
width: 300px;
height: 100px;
padding: 30px;
box-shadow: -30px -30px 0 -25px red,30px 30px 0 -25px green;
}
</style>
<body>
<div class="decorator">段落内容：用box-shadow模拟双色方括号box-shadow: -24px -24px 0 -20px red,24px 24px 0 -20px green; </div>
</body>
（学习视频分享：css视频教程）
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