What are pseudo-class selectors in CSS?

Pseudo-class selectors are a feature of Cascading Style Sheets (CSS) that allow you to define a special state of an element. They are used to target elements not just based on their hierarchy or structure in the document tree, but on characteristics that cannot be targeted with other selector types, like their dynamic state or user interaction.

Here is a brief overview of some commonly used pseudo-classes:

1. :hover - Selects an element when a user hovers over it with a pointing device, usually a mouse.
2. :focus - Selects an element when it has focus, which typically occurs when the user clicks on an input element or navigates to it using the keyboard.
3. :active - Selects an element at the moment the user activates it, for example by clicking a link or a button.
4. :visited - Selects links (<a> elements) that the user has already visited.
5. :link - Selects links that the user has not yet visited.
6. :first-child - Selects an element that is the first child of its parent.
7. :last-child - Selects an element that is the last child of its parent.
8. :nth-child() - Selects an element based on its position in a group of siblings, using a formula or keyword.
9. :nth-of-type() - Similar to :nth-child(), but only counts elements of the same type (tag name).
10. :not() - Selects all elements that do not match the given selector.

Examples of using pseudo-class selectors in CSS:

/* Changes the color of links when hovered over */
a:hover {
    color: blue;
}

/* Styles input fields when they are in focus */
input:focus {
    border: 2px solid green;
}

/* Styles only the first paragraph inside a div */
div p:first-child {
    font-weight: bold;
}

/* Styles the third list item in an ordered list */
ol li:nth-child(3) {
    background-color: yellow;
}

/* Styles paragraphs that are not the first child of their parent */
p:not(:first-child) {
    margin-top: 1em;
}

Pseudo-classes can be combined with other selectors to create more specific and powerful CSS rules. They play a significant role in creating interactive and responsive designs that respond to user actions without the need for JavaScript.

It's important to note that pseudo-classes are part of the CSS specification, and as such, they are supported by all modern web browsers. However, the level of support for certain newer pseudo-classes may vary between browsers, so it's always a good idea to check browser compatibility when using them.