What are the functions and differences between pseudo-elements and pseudo-classes?

Pseudo elements and pseudo classes are two important concepts in CSS, and they play a very key role in web design. This article will introduce in detail the differences and functions of pseudo-elements and pseudo-classes, and provide specific code examples.

1. Pseudo elements
Pseudo elements are a way to virtually create elements in a document. Pseudo elements are represented by double colons (::). Common pseudo elements include the following:

1. ::before pseudo element:
   ::before pseudo element is used for selecting elements. Create a dummy element in front. The content of pseudo elements can be set through the content attribute.

Sample code:

<style>
    p::before {
        content: "前方的虚拟元素";
        color: red;
    }
</style>
<p>这是正常的段落。</p>

With the above code, a red text "dummy element in front" will be inserted before the paragraph element.

2. ::after pseudo-element: The
   ::after pseudo-element is used to create a virtual element behind the selected element. You can also use the content attribute to set the content of the pseudo-element.

Sample code:

<style>
    p::after {
        content: "后方的虚拟元素";
        color: blue;
    }
</style>
<p>这是正常的段落。</p>

With the above code, a blue text "rear virtual element" will be inserted after the paragraph element.

2. Pseudo-class
Pseudo-class is used to select the changed state of a specific element, or to select an element at a specific position. Pseudo-classes are represented by a single colon (:). Common pseudo-classes include the following:

1. :hover pseudo-class:
   :hover pseudo-class is used to select the element when the mouse is hovering. status at the time.

Sample code:

<style>
    a:hover {
        color: red;
    }
</style>
<a href="#">鼠标悬停时变为红色</a>

The above code will set the font color to red when the mouse is hovering over the link.

2. :active pseudo-class:
   :active pseudo-class is used to select the state in which the element is activated, usually the state when the mouse is pressed on the element but has not been released.

Sample code:

<style>
    button:active {
        background-color: blue;
    }
</style>
<button>按钮</button>

The above code will set the background color to blue when the button is pressed but not released.

3. :nth-child pseudo-class:
   :nth-child pseudo-class is used to select the specified child element of an element.

Sample code:

<style>
    li:nth-child(2) {
        color: red;
    }
</style>
<ul>
    <li>第一个元素</li>
    <li>第二个元素</li>
    <li>第三个元素</li>
</ul>

The above code will select the second li element in the list and set its text color to red.

Summary:
Pseudo elements and pseudo classes play an important role in CSS. Pseudo-elements are used to create virtual elements before and after the selected element, while pseudo-classes are used to select the state or position of a specific element. Through the flexible use of pseudo-elements and pseudo-classes, we can control the style of web pages more abundantly and finely.

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