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what are css3 pseudo elements

青灯夜游Original: 2022-01-06 11:08:542770browse

In CSS3, pseudo-element is a keyword appended to the end of the selector. Its literal meaning is "fake element" or "disguise element", which is actually a virtual element; pseudo-element is mainly used for Create some elements that are not in the DOM tree and add styles to them.

The operating environment of this tutorial: Windows 7 system, CSS3&&HTML5 version, Dell G3 computer.

css3 pseudo-element

The literal meaning of pseudo-element is "fake element" or "disguise element". In fact, it can be understood this way. Pseudo elements are actually virtual elements. Elements that do not exist (in code form), and you cannot find them in the document, so pseudo elements are virtual elements.

Pseudo element is a keyword appended to the end of the selector. It is mainly used to create some elements that are not in the DOM tree and add styles to them.

With pseudo-elements you can define styles for specific parts of the selected element without resorting to the element's ID or class attributes. 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.

selector::pseudo-element {
    property: value;
}

Among them, selector is the selector, pseudo-element is the name of the pseudo element, property is the attribute in CSS, and value is the value corresponding to the attribute.

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:

##::first -linep::first-lineMatch the first line of content in each

element

::selectionp::selectionMatch the part of the element selected by the user::placeholderinput::placeholderMatch the placeholder attribute of each form input box (such as )

1. ::after

伪元素 ::after 能够在指定元素的后面插入一些内容，在 ::after 中需要使用 content 属性来定义要追加的内容，而且在 ::after 中必须定义 content 属性才会生效（没有需要插入的内容时可以将 content 属性的值定义为空""）。

下面通过一个示例来演示伪元素 ::after 的使用：

<!DOCTYPE html>
<html>
<head>
    <style>
        p.one::after {
            content:"";
            display: inline-block;
            width: 50px;
            height: 10px;
            background: blue;
        }
        p.two::after {
            content:"要插入的内容";
            color: red;
            font-size: 6px;
        }
        p.three::after {
            content: url(&#39;./smiley.gif&#39;);
            position: relative;
            top: 8px;
        }
    </style>
</head>
<body>
    <p class="one">伪元素 ::after</p>
    <p class="two">伪元素 ::after</p>
    <p class="three">伪元素 ::after</p>
</body>
</html>

运行结果如下图所示：

what are css3 pseudo elements

2. ::before

伪元素 ::before 能够在指定元素的前面插入一些内容。与 ::after 相似，::before 中也需要使用 content 属性来定义要追加的内容，而且在 ::before 中必须定义 content 属性才会生效（没有需要插入的内容时可以将 content 属性的值定义为空""）。

下面通过一个示例来演示伪元素 ::before 的使用：

<!DOCTYPE html>
<html>
<head>
    <style>
        p.one::before {
            content:"";
            display: inline-block;
            width: 50px;
            height: 10px;
            background: blue;
        }
        p.two::before {
            content:"要插入的内容";
            color: red;
            font-size: 6px;
        }
        p.three::before {
            content: url(&#39;./smiley.gif&#39;);
            position: relative;
            top: 8px;
        }
    </style>
</head>
<body>
    <p class="one">伪元素 ::before</p>
    <p class="two">伪元素 ::before</p>
    <p class="three">伪元素 ::before</p>
</body>
</html>

运行结果如下图所示：

what are css3 pseudo elements

3. ::first-letter

伪元素 ::first-letter 用来设置指定元素中内容第一个字符的样式，通常用来配合 font-size 和 float 属性制作首字下沉效果。需要注意的是，伪元素 ::first-letter 仅可以用于块级元素，行内元素想要使用该伪元素，则需要先将其转换为块级元素。

下面通过示例来演示伪元素 ::first-letter 的使用：

<!DOCTYPE html>
<html>
<head>
    <style>
        p::first-letter{
            font-size: 2em;
            color: blue;
        }
    </style>
</head>
<body>
    <p>伪元素 ::first-letter</p>
</body>
</html>

运行结果如下图所示：

what are css3 pseudo elements

4. ::first-line

伪元素 ::first-line 用来设置指定元素中内容第一行的样式，与 ::first-letter 类似，伪元素 ::first-line 也仅可以用于块级元素，行内元素想要使用该伪元素，则需要先将其转换为块级元素。

下面通过示例来演示伪元素 ::first-line 的使用：

<!DOCTYPE html>
<html>
<head>
    <style>
        p::first-line{
            font-size: 1.5em;
            color: blue;
            font-weight: bold;
        }
    </style>
</head>
<body>
    <p>伪元素 ::first-line 用来设置指定元素中内容第一行的样式，与 ::first-letter 类似，伪元素 ::first-line 也仅可以用于块级元素，行内元素想要使用该伪元素，则需要先将其转换为块级元素。</p>
</body>
</html>

运行结果如下图所示：

what are css3 pseudo elements

5. ::selection

伪元素 ::selection 用来设置对象被选中时的样式，需要注意的是，伪元素 ::selection 中只能定义元素被选中时的 color、background、cursor、outline 以及 text-shadow（IE11 尚不支持定义该属性）等属性。

下面通过示例来演示伪元素 ::selection 的使用：

<!DOCTYPE html>
<html>
<head>
    <style>
        p::selection{
            color: red;
            background-color: #CCC;
        }
    </style>
</head>
<body>
    <p>伪元素 ::selection 用来设置对象被选中时的样式，需要注意的是，伪元素 ::selection 中只能定义元素被选中时的 color、background、cursor、outline 以及 text-shadow（IE11 尚不支持定义该属性）等属性。 </p>
</body>
</html>

运行结果如下图所示：

what are css3 pseudo elements

6. ::placeholder

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>

运行结果如下图所示：

what are css3 pseudo elements

（学习视频分享：css视频教程）

Pseudo-elements	Examples	Example description
::after	p::after	Insert content after each element
::before	p::before	Insert content before each element
: :first-letter	p::first-letter	Matches the first letter of the content in each element

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