CSS pseudo-selector learning pseudo-element selector analysis

In the previous article "What is a hierarchical selector in CSS? how to use? ", we learned about 4 types of hierarchical selectors. Let's talk about pseudo-selectors. They provide more complex functions, but do not directly correspond to the elements that HTML documents should have. There are two main types of pseudo-selectors: pseudo-elements and pseudo-classes. Let’s first talk about pseudo-element selectors in detail.

Pseudo-element selector

Pseudo-element selector in CSS is to add keywords to the specified CSS selector. Used to describe the styling of a specific part of a specified element. [Recommended learning: css video tutorial]

Through pseudo-elements, developers can define styles for specific parts of the selected element without using the ID or class attributes of the element. For example, through pseudo-elements you can set the style of the first letter in a paragraph, or insert some content before or after the element, etc.

In CSS1 and CSS2, the use of pseudo-elements is the same as pseudo-classes, and a colon : is connected to the selector. However, in CSS3, the use of single colon for pseudo elements was changed to double colon :: to distinguish pseudo classes and pseudo elements. Therefore, it is recommended to use double colons instead of single colons when using pseudo-elements.

The syntax structure is as follows:

/* CSS3 语法 */
选择器::伪元素 {
  属性 : 属性值;
}
/* CSS2 过时语法 (仅用来支持 IE8) */
选择器:伪元素 {
  属性 : 属性值;
}

should now use two colons, unless you are also compatible with IE8.

Note: Only one pseudo-element can be used in a selector, and the pseudo-element must follow the selector. According to the latest W3C specification, you should use double colon :: instead of single colon : when defining pseudo-elements to distinguish pseudo-classes and pseudo-elements. However, since the old version of the W3C specification did not make a special distinction, most browsers currently support both single colon and double colon methods to define pseudo-elements.

CSS provides a series of pseudo-elements, as shown in the following table:

::before和::after伪元素

::before 伪元素 的作用是作为定位的HTML元素的第一个子级元素，::after ** 伪元素** 的作用是作为定位的 HTML元素的最后一个子级元素。

如下示例代码展示了::before和::after伪元素的用法：

nbsp;html>



    <meta>
    <meta>
    <meta>
    <title>::before和::after伪元素</title>
    <style>
        p::before {
            content: "♥";
        }

        p::after {
            content: "♥";
        }
    </style>



    <p>这是一段测试内容</p>

代码运行结果如下图所示：

如上述示例代码所示，::before伪元素和::after伪元素通常会配合content属性来为该元素增加装饰内容。

content属性用于在元素的::before伪元素和::after伪元素中插入内容。该属性具有的值如下所示：

• none：不会产生伪类元素。

• normal：::before伪元素和::after伪类元素中会被视为 none。

• text：文本内容。

• url：格式为url()，指定一个外部资源（比如图片）。如果该资源或图片不能显示，它就会被忽略或显示一些占位。

::first-letter和::first-line伪元素

::first-letter和::first-line伪元素分别匹配文本的第一个字和第一行的样式内容。示例代码如下：

nbsp;html>



    <meta>
    <meta>
    <meta>
    <title>::first-letter和::first-line伪元素</title>
    <style>
        /* 匹配第一行 */
        p::first-line {
            background-color: lightcoral;
        }

        /* 匹配第一个字 */
        p::first-letter {
            font-size: 130%;
        }
    </style>



    <p>我如果爱你——绝不像攀援的凌霄花，借你的高枝炫耀自己；</p>

代码运行结果如下图所示：

值得注意的是::first-letter和::first-line伪元素可以使用的CSS属性是有限制的。

::first-letter选择器可以设置的CSS属性：

• font属性

• color属性

• background属性

• margin属性

• padding属性

• border属性

• text-decoration属性

• vertical-align属性

• text-transform属性

• line-height属性

• float属性

• clear属性

::first-line选择器可以设置的CSS属性：

• font属性

• color属性

• background属性

• word-spacing属性

• letter-spacing属性

• text-decoration属性

• vertical-align属性

• text-transform属性

• line-height属性

• clear属性

::selection伪元素

::selection伪元素的作用是匹配用户在HTML页面选中的文本内容（比如使用鼠标或其他选择设备选中的部分）设置高亮效果。如下示例代码展示了::selection伪元素的用法：

nbsp;html>



    <meta>
    <meta>
    <meta>
    <title>::first-letter和::first-line伪元素</title>
    <style>
        p::selection {
            color: gold;
            background-color: red;
        }
    </style>



    <p>我如果爱你——绝不像攀援的凌霄花，借你的高枝炫耀自己；</p>

代码运行结果如下图所示：

::placeholder伪元素

伪元素 ::placeholder 用来设置表单元素（、 元素）的占位文本（通过 HTML 的 placeholder 属性设置的文本），示例代码如下：    

<!DOCTYPE html>
<html>
<head>
    <style>
        input.text::placeholder{
            color: red;
            background-color: #CCC;
        }
    </style>
</head>
<body>
    <input placeholder="请输入一段文本">未使用伪元素 ::placeholder<br>
    <input placeholder="请输入一段文本" class="text">使用伪元素 ::placeholder 的效果
</body>
</html>

代码运行结果如下图所示：

（学习视频分享：web前端入门）

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