CSS selectors and specificity calculation rules

                  /* Specificity: 0,0,0,1 - Element selector */
p {
  color: black;
}

/* Specificity: 0,0,1,0 - Class selector */
.text {
  color: blue;
}

/* Specificity: 0,1,0,0 - ID selector */
#main-text {
  color: green;
}

/* Specificity: 0,0,1,1 - Class + Element */
p.highlight {
  color: yellow;
}

/* Specificity: 0,1,1,1 - ID + Class + Element */
#main p.highlight {
  color: orange;
}

/* Specificity: 1,0,0,0 - Inline style (written in HTML) */
/* <p style="color: red;">Highest specificity</p> */

/* !important overrides everything (avoid when possible) */
.important-text {
  color: purple !important; /* Wins over inline styles */
}

/* Attribute selectors - Specificity: 0,0,1,0 */
[data-theme="dark"] {
  background: #222;
}

[type="submit"] {
  background: blue;
}

[class*="btn"] { /* Contains "btn" */
  padding: 0.5rem 1rem;
}

[href^="https"] { /* Starts with https */
  color: green;
}

[href$=".pdf"] { /* Ends with .pdf */
  background: url(pdf-icon.png) no-repeat;
}

[lang|="en"] { /* Exact or with hyphen */
  quotes: '"' '"';
}

/* Pseudo-class selectors - Specificity: 0,0,1,0 */
a:hover {
  text-decoration: underline;
}

input:focus {
  outline: 2px solid blue;
}

li:first-child {
  font-weight: bold;
}

li:last-child {
  border-bottom: none;
}

li:nth-child(odd) {
  background: #f5f5f5;
}

li:nth-child(3n+1) { /* 1st, 4th, 7th... */
  color: red;
}

p:not(.exclude) {
  margin-bottom: 1rem;
}

/* Pseudo-element selectors - Specificity: 0,0,0,1 */
p::before {
  content: "→ ";
}

p::after {
  content: " ←";
}

::selection {
  background: yellow;
  color: black;
}

input::placeholder {
  color: #999;
  opacity: 1;
}

/* Modern pseudo-classes */
:is(h1, h2, h3) {
  /* Specificity = highest selector (h1) = 0,0,0,1 */
  font-family: 'Heading Font', sans-serif;
}

:is(.btn, #submit-btn) {
  /* Specificity = highest (#submit-btn) = 0,1,0,0 */
  padding: 0.5rem 1rem;
}

:where(h1, h2, h3) {
  /* Specificity = 0,0,0,0 - Zero specificity! */
  margin-top: 0;
}

/* :has() - parent selector */
.card:has(img) {
  display: grid;
  grid-template-columns: 200px 1fr;
}

form:has(input:invalid) {
  border-color: red;
}

/* Combinators */
/* Descendant - Specificity: 0,0,0,2 */
div p {
  color: blue;
}

/* Child - Specificity: 0,0,0,2 */
ul > li {
  list-style: none;
}

/* Adjacent sibling - Specificity: 0,0,0,2 */
h2 + p {
  font-size: 1.2rem;
}

/* General sibling - Specificity: 0,0,0,2 */
h2 ~ p {
  color: gray;
}

/* State pseudo-classes */
input:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

input:checked + label {
  font-weight: bold;
}

option:checked {
  background: lightblue;
}

:target {
  background: yellow; /* Element targeted by URL fragment */
}

/* Form validation pseudo-classes */
input:valid {
  border-color: green;
}

input:invalid {
  border-color: red;
}

input:required {
  border-left: 3px solid blue;
}

input:optional {
  border-left: 3px solid gray;
}

input:in-range {
  border-color: green;
}

input:out-of-range {
  border-color: red;
}

/* Structural pseudo-classes */
:root {
  --main-color: blue; /* Root element */
}

:empty {
  display: none; /* Elements with no children */
}

tr:nth-child(even) {
  background: #f9f9f9;
}

li:only-child {
  list-style: none;
}

/* Specificity comparison examples */
/* Specificity: 0,0,1,0 - Wins */
.btn { color: blue; }

/* Specificity: 0,0,0,1 - Loses */
button { color: red; }

/* Specificity: 0,1,0,0 - Wins */
#submit { color: green; }

/* Specificity: 0,0,2,1 - Loses */
button.btn.primary { color: yellow; }

/* Best practice: avoid overly specific selectors */
/* Bad - too specific */
body div.container ul.nav li.nav-item a.nav-link {
  color: blue;
}

/* Good - just enough specificity */
.nav-link {
  color: blue;
}

/* Using :where() for zero-specificity base styles */
:where(h1, h2, h3, h4, h5, h6) {
  margin: 0;
  font-weight: normal;
}

/* Easy to override later */
h1 {
  font-size: 2rem; /* 0,0,0,1 beats :where's 0,0,0,0 */
}

/* Custom data attribute selectors */
[data-size="small"] { font-size: 0.875rem; }
[data-size="medium"] { font-size: 1rem; }
[data-size="large"] { font-size: 1.25rem; }

[data-state~="active"] { /* Word in space-separated list */
  background: blue;
}