How to achieve the effect of adding shadow to element blocks in css

In CSS, you can use the box-shadow attribute to achieve the effect of adding shadow to an element block. The box-shadow attribute can realize the border shadow effect and apply the shadow to the box element. The syntax is "box-shadow: Horizontal shadow Vertical shadow Blur radius Expansion radius Shadow color Projection mode"; if the projection mode is set to "inset", inner shadow can be achieved.

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 effect of adding shadow to an element block.

The box-shadow property can apply shadow to the text box, and can set the pixel length, width and blur distance of the shadow as well as the color of the shadow.

box-shadow can add shadows to box elements, supporting adding one or more.

box-shadow: X轴偏移量 Y轴偏移量 阴影模糊半径 阴影扩展半径 阴影颜色 投影方式;

Note: The boxShadow property adds one or more drop-down shadows to the box. This property is a comma-separated list of shades, each shade specified by 2-4 length values, an optional color value, and an optional inset keyword. The value for omitted length is 0.

Note: inset can be written in the first or last parameter, and other positions are invalid.

• X-axis offset and Y-axis offset values (horizontal shading and vertical shading)

  X-axis offset and Y-axis offset The axis offset value can be set to a negative number

• Shadow blur radius:

  This parameter is optional and the value can only be positive. If the value is 0, it means that the shadow has no blur effect. The larger the value, the blurr the edge of the shadow.

• Shadow expansion radius:

  This parameter is optional. The value can be positive or negative. If the value is positive, the entire shadow will be extended. Expand, otherwise if the value is negative, reduce it.

Usage of box-shadow attribute

1. The horizontal and vertical offset can also be 0. Shadow

If the offset-x or offset-y value is 0, the shadow is behind the element. At this time, giving the blur-radius value or spread value can produce a shadow effect.

Example:

The first div produces a shadow effect by setting blur-radius.

The second div produces a shadow effect by setting a positive spread value.

The third div produces a shadow effect by setting a negative spread value.

But there is one thing to note: expanded shadows must be used in conjunction with the shadow blur radius.

I personally think it should be used without coordination, but it is impossible to only set the extended shadow, because the values of the extended shadow and shadow blur can both be positive. If there is only extended shadow, it will be parsed as a blur shadow by the browser, so it can also be simply understood as "expanded shadow must be used in conjunction with the shadow blur radius". If only extended shadow is used, it can be written as: box-shadow:0 0 0 1px;.

2. Set the horizontal and vertical offset to get the shadow effect

outset situation: horizontal and vertical offset Move to 0, but do not set blur and spread, and the shadow cannot be seen, because the perimeter of the box-shadow is the same asborder-boxat this time, so the shadow can be displayed by setting the offset.

inset situation: the horizontal and vertical offset is 0, blur and spread are not set, and the shadow is also not visible, because the perimeter of box-shadow is the same aspadding-boxat this time, the same You can make the shadow show through by setting an offset.

Example:

3. Projection method

The default projection method is Outset, that is, external projection, can be set to inset to project inward.

Example: The first div defaults to outset, the second div sets inset, the third div sets two shadows at the same time to better see the relationship between outset and inset, and the fourth div can see the inset shadow. Above the background, below the content.

  
 
 
 
inset阴影在背景之上，内容之下
 

4. If the element also specifies the border-radius attribute, the shadow will have the same rounded corners.

5. Classic example

An example in w3c. http://www.w3.org/TR/css3-background/#the-box-shadow

##Visible:

• 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。

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

  
border
 
box-shadow
 
多重
box-shadow
 

用spread实现双色方括号

  
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; 
 

（学习视频分享：web前端）

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