Pseudo-classes are styles used to define element states, such as:hover and:focus; pseudo-elements create virtual content, such as::before and::after. A pseudo-class represents the state or positional relationship of an element, such as a button being clicked or a link being accessed; a pseudo-element is used to style a part of the content or insert virtual content, such as a first line of the paragraph or a placeholder for the input box. Pay attention to browser compatibility when using it. Pseudo-elements usually need to be matched with content attributes and avoid overuse to maintain style maintainability.
CSS pseudo-classes and pseudo-elements are two features often used by developers to more flexibly select and style specific parts of a page. They look a bit like ordinary classes or elements, but are not actually part of the document structure, but are applied dynamically through CSS.
What are pseudo-classes?
Pseudo-classes are used to define the style of an element in a certain state , such as when a link is clicked, mouse hovered, or form elements gain focus.
Common examples include:
-
:hover: When the user hovers over the element. -
:active: When the element is activated (such as when the button is clicked). -
:focus: When the element gets focus (such as the input box is selected). -
:nth-child(): Matches the nth child element under the parent element.
To give a simple example, if you want a button to change color when the mouse hovers, you can write it like this:
button:hover {
background-color: #007bff;
}These states are not written in HTML, but are triggered dynamically based on user interaction or browser status.
What are pseudo-elements?
Pseudo-elements are used to create some "virtual" elements that are not in the document structure. They are usually used to style certain content without modifying HTML code.
Common pseudo-elements are:
-
::beforeand::after: You can insert content before and after the element content. -
::first-line: The first line of the styled text paragraph. -
::placeholder: Controls the style of placeholder text in the input box. -
::selection: Defines the background color when the user selects text.
For example, if you want to add a decorative symbol before each paragraph, you can do this:
p::before {
content: "? ";
font-weight: bold;
} Note: Pseudo-elements use double colons :: to distinguish pseudo-categories, but single colons are also accepted in older browsers.
The difference between pseudo-class and pseudo-element
Simply put:
- A pseudo-class describes the state or positional relationship of an element, such as whether it has been visited or whether it is a child element.
- A pseudo-element describes the content part of an element, usually a visual extension, such as adding before and after content or modifying part of the content.
For example:
-
a:visitedis a pseudo-class, indicating that the link has been visited. -
p::first-lineis a pseudo-element that represents the first line of the paragraph.
Some precautions in actual use
- Pay attention to browser compatibility when using pseudo-classes, especially more complex expressions like
:nth-child(). - Pseudo-elements need to be used with
contentattribute to be displayed (except for a few exceptions such as::first-line). - Do not abuse pseudo-classes and pseudo-elements, otherwise the style may be difficult to maintain.
- In responsive design, pseudo-elements are often used to generate auxiliary content, such as icons, prompt information, etc.
Basically that's it. The key to understanding pseudo-classes and pseudo-elements is that pseudo-classes are "states" and pseudo-elements are "virtual content". Mastering them allows you to create richer interactions and visual effects without changing HTML.
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