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What are CSS pseudo-elements? Detailed explanation of CSS pseudo-element usage
What are CSS pseudo-elements? Detailed explanation of CSS pseudo-element usage
什么是伪元素
伪元素表示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。
因为伪元素并不在文档树中，所以通过JS，也抓取不到伪元素，也不能给它绑事件。
语法
伪元素以::开头。
在CSS1和CSS2中，伪元素和伪类一样，都是用:开头。但在CSS3中，伪元素以::开头，用以和伪类进行区分。
IE8不支持::。因此如果要兼容IE8，只能用:。
常见的伪元素
::before
在当前元素的内容的前面插入一个子元素。插入的元素为内联元素。
需要注意的是，使用::before时， 必须使用content来指定子元素的内容。
例如：
.element::before {
content: "Note: "; /* 字符串 */
}
.element::before {
content: attr(title); /* 元素的title属性 */
}
.element::before {
content: url(path/to/image.png); /* 一个图片 */
}
.element::before {
content: "\201C"; /* Unicode转义的一个字符 */
}更详细的介绍见这里
::after
在当前元素的内容的后面插入一个子元素。
其他和 ::before类似。
::first-line
选择当前元素的第一行。
需要注意的是，其只作用于块级元素[注1]。
对::first-line只能使用下面的样式
Font: font, font-style, font-variant, font-weight, font-size, line-height和font-family. Background: background, background-color, background-image, background-position, background-repeat, background-size和background-attachment text-decoration, text-transform, letter-spacing和word-spacing
Copy after login因此，对::first-line使用margin和padding是无效的。
更详细的介绍见这里。
::first-letter
选择第一个字母。
更详细的介绍见这里。::selection
选中当前元素中，选中的文字。
selection.png
对::selection的只能使用 color, background-color和text-shadow这几个属性。
什么时候用伪元素
HTML 标签的目的，就是为了展示内容信息。非内容信息要使用伪元素。
具体的使用场景是图标和清除浮动。
所谓的非内容信息，指的是一些对内容进行修饰的信息。
如下图，登录按钮左边的图标是对登录的说明，为非内容信息。
登录按钮.png
如下图，用红框框起来的小图标表示，点击左边的链接，会在新窗口打开。也是非内容信息。
外链.png
demos
那些 CSS 偽元素可以幫你做的 10 個效果
基于单个 p 的 CSS 绘图
注
块级元素指display的值为block, inline-block, table-cell, table-caption或 list-item中的一个的。
【相关推荐】
1. 免费css在线视频教程
2. css在线手册
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