What are pseudo-classes and which are common?

Question

Accepted Answer

A **pseudo-class** (single colon `:`) selects elements based on their **state** or **position** — something not expressible by a plain class, like "while hovered" or "the first child."

## State pseudo-classes (interaction)

```css
a:hover    { color: blue; }      /* mouse is over it */
a:focus    { outline: 2px solid; } /* keyboard/tab focus — keep for accessibility! */
button:active   { transform: scale(0.98); } /* while being pressed */
button:disabled { opacity: 0.5; } /* disabled form controls */
input:checked   { } /* checked checkbox/radio */
input:focus-visible { } /* focus only when keyboard-navigating (not mouse) */
```

The order for links matters (LVHA): `:link`, `:visited`, `:hover`, `:active`.

## Structural pseudo-classes (position)

```css
li:first-child  { } /* first li among its siblings */
li:last-child   { } /* last one */
li:nth-child(2) { } /* the 2nd */
li:nth-child(odd)  { } /* 1st, 3rd, 5th... — zebra striping */
li:nth-child(3n)   { } /* every 3rd */
p:only-child    { } /* a p that's the sole child */
```

`:nth-child()` is powerful — `2n` (even), `2n+1` (odd), `3n` (every third), etc.

## Form & modern pseudo-classes

```css
input:required  { }
input:invalid   { border-color: red; }
:is(h1, h2, h3) { }  /* matches any — shortens selector lists */
.card:has(img)  { }  /* the "parent selector": a card that CONTAINS an img */
:not(.active)   { }  /* everything except .active */
```

## Pseudo-class vs pseudo-element

Pseudo-classes use **one** colon (`:hover`, a *state*); pseudo-elements use **two** (`::before`, a *generated part*). Don't confuse them.

## Why it matters

Pseudo-classes let you style interaction (hover/focus — essential for usable, accessible UI) and structure (striping, first/last) without extra markup or JavaScript.

Modern ones (`:has`, `:is`, `:focus-visible`) solve problems that previously required JS or repetitive selectors.