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."
{ : blue; }
{ : solid; }
{ : (); }
{ : ; }
{ }
{ }
The order for links matters (LVHA): :link, :visited, :hover, :active.
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.
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-classes use one colon (:hover, a state); pseudo-elements use two (::before, a generated part). Don't confuse them.
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.