What are CSS pseudo-classes and pseudo-elements and how do they differ?

CSS pseudo-classes and pseudo-elements: Definitions

CSS pseudo-classes are selectors used to specify a particular state of an element. For example, when a user interacts with an element, such as hovering over it or when it gains focus, we can use pseudo-classes to change the styling of these elements. Pseudo-classes are denoted by a single colon (e.g., :hover, :focus).

CSS pseudo-elements are used to style elements that do not exist in the document tree. Pseudo-elements allow us to style specific parts of an element, such as the first line or the first letter. Pseudo-elements are denoted by double colons (e.g., ::before, ::after), which was introduced in CSS3 to distinguish them from pseudo-classes. For instance, ::before and ::after pseudo-elements can add new content before or after the element's content, typically used with the content property.

Their Differences

1. Syntax Differences:

   • Pseudo-classes use a single colon (e.g., :hover) to denote states.
   • Pseudo-elements use double colons (e.g., ::before) to style specific content.

2. Functional Differences:

   • Pseudo-classes define specific states of elements (e.g., :hover indicates a hover state), focusing on state changes.
   • Pseudo-elements create parts that do not exist in the document tree, effectively creating virtual elements via CSS. They focus on content before/after or specific parts for design and layout.

Practical Examples

Pseudo-class Examples:

css
a:hover {
    color: red; /* When hovering over the link, it turns red */
}
input:focus {
    border: 2px solid blue; /* When the input field gains focus, it displays a blue border */
}

Pseudo-element Examples:

css
p::first-line {
    font-weight: bold; /* First line text is bold */
}
div::before {
    content: "Note: ";
    color: red; /* Adds red "Note: " text before the div's content */
}

Through these examples, we can see that CSS pseudo-classes and pseudo-elements are widely used and valuable in web design, each serving its specific purpose and playing an important role.

CSS pseudo-classes and pseudo-elements are methods used to specify special states or specific parts of elements on a page. Although similar, they differ in purpose and definition.

Pseudo-classes

Pseudo-classes are used to define specific states of an element. For example, the style when a user hovers over an element, or whether an element has been visited. Pseudo-classes are denoted by a single colon :. Common pseudo-classes include:

• :hover — Applies styles when the user hovers the mouse over an element.
• :active — Applies styles when the user clicks and holds the element.
• :visited — Used for links that have been visited.
• :first-child — Selects the first child of a parent element.

For example, if we want to change the color of a link when the mouse hovers over it, we can write:

css
a:hover {
    color: red;
}

Pseudo-elements

Pseudo-elements are used to specify particular parts of an element on the page. They are denoted by double colons ::, which distinguishes them from pseudo-classes in CSS3. Common pseudo-elements include:

• ::before — Inserts content before the element's content.
• ::after — Inserts content after the element's content.
• ::first-letter — Selects the first letter of a text block.
• ::first-line — Selects the first line of a text block.

For example, if we want to add a decorative quotation mark at the beginning of each paragraph, we can use the ::before pseudo-element:

css
p::before {
    content: "“";
    color: blue;
}

Key Differences

• Syntax: Pseudo-classes use a single colon :, while pseudo-elements use double colons ::.
• Definition: Pseudo-classes describe specific states of an element, whereas pseudo-elements are used to define parts not in the document tree for styling certain content.
• Purpose: Pseudo-classes are often used for styling dynamic states, such as :hover, while pseudo-elements are commonly used for page layout and content decoration, such as ::before.

Understanding these differences helps in effectively using CSS to design and control the styling of web pages.