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A brief discussion on the understanding of css pseudo-classes and pseudo-elements
A brief discussion on the understanding of css pseudo-classes and pseudo-elements
前言
今天主要谈谈css的伪类和伪元素。其实，关于伪类和伪元素，我在小工具CSS 选择器演示 中，写了不少伪类和伪元素，大家可以去看一下！
伪类和伪元素的理解
官方解释
伪类一开始单单只是用来表示一些元素的动态状态，典型的就是链接的各个状态(LVHA)。随后CSS2标准扩展了其概念范围，使其成为了所有逻辑上存在但在文档树中却无须标识的“幽灵”分类。
伪元素则代表了某个元素的子元素，这个子元素虽然在逻辑上存在，但却并不实际存在于文档树中。
我自己的理解：
伪类，类，class是一个类，就类似class这样，表示一些元素的状态的，但是无须标识的分类。
伪元素，就是一个元素，类似p标签元素，等等，逻辑上存在，但是实际并不存在在文档树中。
另外一个简单的理解和区分就是：
伪类前面是一个冒号，为元素前面是两个冒号。E:first-child 伪类，E::first-line为元素。
特殊情况::before 和 ::after 伪元素
before 和after是在元素前面和后面添加一些元素，因此是伪元素，CSS3选择器中已经将其前面变为两个冒号了。请看：w3cselect
问题来了，那么为什么我们平时用before和after前面就一个冒号？
当你看了css2选择器就知道了，老的规范中，伪类和伪元素都用一个冒号，新规范为了区分，伪元素统一用两个“：：”,所以，大家注意了，往后在写css3的时候，为元素要用两个冒号，伪类用一个冒号。
伪类和伪元素的应用
我之前的css常用效果，讲了一些。
像伪元素
::-webkit-scrollbar::selection::-webkit-input-placeholder ::after
等等。
伪元素中用的最多的要数::after和::before了
可以用这两个伪元素做很多效果！
html
<a href="#">haorooms</a>
css
a { position: relative;
display: inline-block; outline: none;
text-decoration: none; color: #000; font-size: 32px;
padding: 5px 10px;}a:hover::before,
a:hover::after { position: absolute; }
a:hover::before
{ content: "\5B"; left: -20px; }
a:hover::after { content: "\5D"; right: -20px; }其中content中可以用attr也可以用url
例如：
a：:after { content:"(" attr(href) ")"; } h1::before { content: url(logo.png); }伪类中用的最多的要数：nth-child()选择器了，关于nth-child()，我之前有篇文章介绍过。
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