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HomeWeb Front-endCSS TutorialWhat 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

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。

What are CSS pseudo-elements? Detailed explanation of CSS pseudo-element usage

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。

What are CSS pseudo-elements? Detailed explanation of CSS pseudo-element usage

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。

What are CSS pseudo-elements? Detailed explanation of CSS pseudo-element usage

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

  1. 块级元素指display的值为block, inline-block, table-cell, table-caption或 list-item中的一个的。

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