Cad iad pseudo-classes agus cén iad na cinn choitianta?

Question

Accepted Answer

Roghnaíonn **pseudo-class** (idirstad amháin `:`) eilimintí bunaithe ar a **staid** nó a **suíomh** — rud nach féidir a chur in iúl le gnáth-aicme, cosúil le "agus é ag foluain" nó "an chéad leanbh."

## Pseudo-classes stáit (idirghníomhaiocht)

```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) */
```

Tá an t-ord tábhachtach do naisc (LVHA): `:link`, `:visited`, `:hover`, `:active`.

## Pseudo-classes struchtúrtha (suíomh)

```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 */
```

Tá `:nth-child()` cumhachtach — `2n` (corr-dhíbh), `2n+1` (neamhchothrom), `3n` (gach tríú ceann), etc.

## Foirmeacha agus pseudo-classes nua-aimseartha

```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

Baineann pseudo-classes úsáid as **an** idirstad (`:hover`, *staid*); bíonn dhá idirstad ar pseudo-elements (`::before`, *páirt ghinte*). Ná mearbhaill iad.

## Cén fáth a bhfuil sé tábhachtach

Ligean do pseudo-classes duit idirghníomhaiocht a gheal (foluain/fócas — riachtanach do UI úsáideora, inchuimsitheach) agus struchtúr (sreanga, an chéad/deireanach) gan bhreise markup ná JavaScript.

Reitíonn na nua-aimseartha (`:has`, `:is`, `:focus-visible`) fadhbanna ar bhain JS nó seanúsáireachta seanroinntear JS ag teastáil uathu.