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Pseudo class elements -- before and after

2022-07-05 10:20:00 【Pursue ~】

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before/after A pseudo class is equivalent to inserting two additional tags inside an element , Its most suitable and recommended application is graphics generation . In some exquisite UI Implementation , Can be simplified HTML Code , Improve readability and maintainability .
Effect use ：
Insert picture description here
Insert picture description here
Small icons like this are mostly used before,after To achieve , Not only simple but also convenient .

1. Basic usage

:before and :after The function of is to specify the element content （ Not the elements themselves ） Insert a containing before or after content Attribute specifies the inline element of the content , The basic usage is as follows ：

#example:before {
    content: "#";
    color: red;
}

#example:after {
    content: "$";
    color: red;
}

Both of these pseudo classes belong to inline elements , But with display:block; Attribute can convert it into a block element , The more common usage is some implementation of styles , There is also the effect of clearing floating ..

2. Style modification

The code is as follows :

<div class="quote">
    <span> Fight tiger </span>
</div>
.quote:before,.quote:after{// Use these two pseudo classes to realize style rendering 
     content:"";
     display:inline-block;
     width:5%;
     margin:5px 1%;
     border-bottom:1px solid blue;
}

3. Remove the floating

The code is as follows ：

<div class="parent">
    <div class="son1"></div>
    <div class="son2"></div>
</div>
<div class="parent2">parent2</div>
//css Code 
.son1{
     width:300px;
     height:200px;
     background-color: lightgray;
     float:left;
}
.son2{
     width:300px;
     height:100px;
     background-color: lightblue;
     float:left;
}
.parent2{
     width:400px;
     height: 400px;
     background-color:blue;
     color:#fff;
     text-align:center;
     line-height:400px;
     font-size:30px;
}

If you add this code to the above code to clear the float, you will achieve a different effect ：

.parent:after{
     content:"";
     display:block;// Set as block element 
     clear:both;   // Use this property to clear the float 
}

::before and ::after Endemic content, Used in css Add content to the logical head or tail of an element in rendering .

These additions will not appear in DOM in , Will not change the content of the document , Do not copy , Just in css Render layer added .

So don't use :before or :after Show meaningful content , Try to use them to show decorative content , For example, icons .
Be careful ： In the use of before and after when content essential .
Be careful ： In the use of before and after when content essential .
Be careful ： In the use of before and after when content essential .

5.content attribute

::before and ::after Must cooperate content Attribute to use ,content Used to define what to insert ,content It must be worth , At least it's empty . By default , Of pseudo class elements display Is the default value inline, Can be set by display:block To change the display .

content Take the following values .

1、string

 Wrap a string in quotation marks , A string will be added to the element content . Such as ：a:after{content:""}

<!DOCTYPE html>
<meta charset="utf-8" />
<style type="text/css">
p::before{
    content: "《";
    color: blue;
}
p::after{
    content: "》";
    color: blue;
}
</style>
<p> Ordinary world </p>

2、attr()

adopt attr() Call the properties of the current element , For example, the picture alt Prompt text or link href The address is displayed .

<style type="text/css">
a::after{
    content: "(" attr(href) ")";
}
</style>
<a href="http://www.cnblogs.com/starof">starof</a>

3、url()/uri()

Used to reference media files .

give an example ：“ Baidu ” Here is a picture , It's given later href attribute .

<style>
a::before{
    content: url("https://www.baidu.com/img/baidu_jgylogo3.gif");
}
a::after{
    content:"("attr(href)")";
}
a{
    text-decoration: none;
}
</style>
---------------------------
<body>
<a href="http://www.baidu.com"> Baidu </a>
</body>

4、counter()

Call counter , You can implement the serial number function without using list elements .

coordination counter-increment and counter-reset Attributes use ：

h2:before { counter-increment: chapter; content: "Chapter " counter(chapter) ". " }

<style>
body{
    counter-reset: section;
}
h1{
    counter-reset: subsection;
}
h1:before{
    counter-increment:section;
    content:counter(section) "、";
}
h2:before{
    counter-increment:subsection;
    content: counter(section) "." counter(subsection) "、";
}
</style>
------------------------------------------------
<body>
<h1>HTML tutorials</h1>
<h2>HTML Tutorial</h2>
<h2>XHTML Tutorial</h2>
<h2>CSS Tutorial</h2>

<h1>Scripting tutorials</h1>
<h2>JavaScript</h2>
<h2>VBScript</h2>

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</body>
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