What is the difference between pseudo-elements and pseudo-classes?

Pseudo elements and pseudo classes are two concepts commonly used in CSS. They are used to control the style and behavior of specific elements in the page. Although they are similar in name, they actually have different functions and ways of using them.

First, let’s take a look at pseudo-elements. Pseudo elements are used to create a virtual element within the selected element and style it. It does this by inserting content before and after the content of the selected element. Pseudo elements start with a double colon (::). Here are some commonly used pseudo elements:

1. ::before: Insert a virtual element before the content of the selected element.
2. ::after: Insert a virtual element after the content of the selected element.
3. ::first-line: Select the first line of text of the selected element.
4. ::first-letter: Select the first letter of the selected element.

For example, we can insert a quotation number before a paragraph through the pseudo element::before:

p::before {
  content: '"';
}

In this way, a quotation number will be displayed before each paragraph.

Next, let’s take a look at pseudo classes. Pseudo-classes are used to select a specific state or position of an element. It is implemented by adding a colon (:) to the selector. Pseudo-classes can be applied to any element, here are some commonly used pseudo-classes:

1. :hover: Applies styles when the mouse is hovering over the element.
2. :focus: Applies the style when the element gains focus.
3. :active: The element applies styles when clicked.
4. :first-child: The first child element in the parent element of the selected element.

For example, we can use the pseudo-class:hover to modify the style of the button to achieve the mouse hover effect:

button:hover {
  background-color: red;
  color: white;
}

When the mouse hovers over the button, the background color of the button will change to red and the text color will change to white.

To sum up, pseudo-element is used to create a virtual element and style it; while pseudo-class is used to select a specific state or position of the element. By understanding the difference between pseudo elements and pseudo classes, we can better master the application of CSS and add various styles and interactive effects to the page.

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