_{—Part of CSS Primer for the AI Era — Selectors}

State selectors let us target elements
based on their current condition
— whether they are
hovered,
focused,
checked,
active,
visited,
empty,
or in some other temporary or structural state.

Where attribute selectors describe properties,
'state selectors' describe moments.

They help us style elements
not just by what they are,
but by what they are doing right now.

State selectors are pseudo‑classes* that
match elements when
they enter a particular condition.

These conditions may be:

• user‑driven
• browser‑driven
• form‑driven
• structural
• temporary
• interactive

State selectors let us adjust
our styling
as an element moves through
different conditions..

These states reflect how a user interacts with an element.

:hover — when the pointer is over the element

• button:hover
  → We use this to provide gentle feedback —
  a visual whisper that says “yes, this is interactive.”

:focus — when the element is focused

• input:focus
  → Essential for accessibility.
  Focus is how keyboard and assistive‑technology users navigate.

:active — when the element is being pressed

• button:active
  → A brief, tactile moment — the press itself.

Forms are full of meaningful conditions.

:checked — for checkboxes and radio buttons

• input[type=“checkbox”]:checked
  → A simple, powerful way to style selected options.

:disabled — when the element cannot be interacted with

• button:disabled
  → This expresses state without relying on classes.

:required and :optional

• input:required
  → We can style fields based on whether they must be filled.

:valid and :invalid

• input:invalid
  → The browser’s built‑in validation becomes a styling hook.

Links have their own lifecycle.

:link — unvisited links

• a:link ← example

:visited — links the user has already visited

a:visited ← example

Browsers protect privacy here, so styling options are intentionally limited.

Example of
Styling Unvisited Links with
a:link
HTML

<a href="https://example.com">Visit Example</a>
<a href="/local-page">Local Page</a>

CSS

a:link {
  color: #0645ad;   /* classic unvisited link color */
  text-decoration: underline;
}

What this does:

The rule applies only to links the visitor has not yet visited.

Once the link is visited, the browser switches to a:visited instead.

This gives us a clean way to differentiate unvisited and visited states without adding classes.

Some states describe the element’s position or content.

:first-child and :last-child

• li:first-child ← example.
  Targets the very first child inside its parent container (such as a <ul> or <ol>).

:empty — elements with no children and no text

• div:empty ← example
  Useful for layout gaps or placeholder styling.

:not() — negation

• button:not([disabled]) ← example.
  This lets us express intent by exclusion.

State selectors let us respond to:

• interaction
• validation
• accessibility
• structure
• temporary conditions
• browser logic

They help us build interfaces that feel alive, responsive, and intentional —
without adding extra classes or JavaScript.

They also give us a vocabulary for guiding AI:

“Style this element when it is in this condition.”

This is the heart of state‑based styling.

We reach for state selectors when:

• the browser already knows the condition
• the condition is temporary
• the condition is user‑driven
• the condition is structural
• adding a class would be redundant

Examples:

• :hover for interaction
• :focus for accessibility
• :checked for toggles
• :invalid for validation
• :empty for layout cleanup

These selectors let us style based on behavior,
not just structure.

We avoid state selectors when:

• the condition is not expressible as a browser state
• the condition depends on logic, not structure
• the condition is long‑lived and should be expressed semantically
• a class would be clearer

For example, “selected tab” is usually a class, not a pseudo‑class.

AI‑Steering Cue:

“Use a state selector — 
style the element when it enters this condition.”

This helps us guide AI
toward behavior‑based
styling rather than structural guessing.

• Suggested Next Reading: Pseudo‑elements — Styling Parts of an Element

_{Tony de Araujo —New York | Lisbon}

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