Pseudo Classes

Pseudo-Classes are special selectors used to define a specific state of an element or select elements based on their position in the HTML structure. They allow you to apply styles that don't depend on the explicit presence of a class or ID in the HTML.

Pseudo-classes are indicated by a single colon (:) followed by the name of the pseudo-class.

State Pseudo-Classes (User Interaction)

These are the most common pseudo-classes. They allow you to change the appearance of an element based on user interaction or the element's current condition.

Pseudo-Class	Description	Example Use Case
:hover	The element is being hovered over by the cursor.	Change button background when the mouse is over it.
:active	The element is clicked or pressed by the user.	Briefly change a link color while it's being clicked.
:focus	The element has keyboard focus (e.g., inputs, links).	Add a visible outline to an input field when a user types in it.
:visited	A link (<a>) that the user has already visited.	Change the color of already-visited links.
:link	An unvisited link (<a>).	Set the initial color of links.
:checked	A radio button or checkbox is selected.	Style a checkbox label when the box is checked.

Example: Styling a Button State

styles.css

/* Base style for the button */
.my-button {
  background-color: #3498db;
  color: white;
  padding: 10px 20px;
  transition: background-color 0.3s; /* Smooth visual change */
}

/* Style when the user hovers over the button */
.my-button:hover {
  background-color: #2980b9; /* Slightly darker */
}

/* Style when the user is actively clicking the button */
.my-button:active {
  background-color: #e74c3c; /* Red color flash */
}

Structural Pseudo-Classes (Position)

These selectors target elements based on their placement or numerical order within a parent container.

Simple Position Selectors

These are useful for targeting the beginning or end of a sequence of elements.

Selector	Description	Example Use Case
:first-child	Selects the element if it is the very first child of its parent.	Remove the top margin from the first paragraph in a container.
:last-child	Selects the element if it is the very last child of its parent.	Remove the bottom border from the last item in a list.
:only-child	Selects the element if it is the sole child of its parent.	Center a menu only if it has just one link.

The nth Selectors

The nth-child() and nth-of-type() pseudo-classes are extremely powerful as they accept an argument to create complex patterns.

Selector	Argument	Description
:nth-child(n)	odd, even, 2n, 3n+1	Selects elements based on their position among all siblings, regardless of tag type.
:nth-of-type(n)	odd, even, 2n	Selects elements based on their position among siblings of the same type (tag).

nth-child vs. nth-of-type

Use :nth-of-type() when you want to style every other element of the same tag (e.g., alternating colors for table rows, tr:nth-of-type(odd)). Use :nth-child() when the element must be in a specific numerical position relative to all children, even if they are different tags.

Specificity Score

Pseudo-Classes have the same medium specificity as Class and Attribute Selectors.

Selector Type	Specificity Score
Pseudo-Classes	(0, 0, 1, 0)

A score of 10 means they can combine effectively with classes without creating specificity nightmares.

Interactive Pseudo-Class Demo

In this demo, observe how :hover changes the button color and how :nth-child creates a zebra-stripe pattern on the list items.