Pseudo-classes

CSS Pseudo-classes

Target elements based on their state, position, or relationship. Pseudo-classes enable dynamic styling without JavaScript.

💻 Pseudo-class Basics

/* Basic syntax */
selector:pseudo-class {
    property: value;
}

/* Examples */
a:hover {
    color: red;
}

input:focus {
    border-color: blue;
}

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

🖱️ User Action Pseudo-classes

:hover

/* Mouse over element */
.button:hover {
    background: #0056b3;
}

a:hover {
    text-decoration: underline;
}

/* Nested hover */
.card:hover .card-title {
    color: #007bff;
}

/* Only on hover-capable devices */
@media (hover: hover) {
    .button:hover {
        background: #0056b3;
    }
}

:active

/* Element being clicked */
.button:active {
    transform: scale(0.95);
}

a:active {
    color: red;
}

/* Touch devices benefit from :active */

:focus

/* Element has focus (keyboard/mouse) */
input:focus {
    border-color: #007bff;
    outline: none;
    box-shadow: 0 0 0 3px rgba(0,123,255,0.25);
}

button:focus {
    outline: 2px solid #007bff;
    outline-offset: 2px;
}

/* Textarea focus */
textarea:focus {
    border-color: #007bff;
}

:focus-visible

/* Focus from keyboard only */
button:focus {
    outline: none;  /* Remove for all focus */
}

button:focus-visible {
    outline: 2px solid #007bff;  /* Only keyboard focus */
    outline-offset: 2px;
}

/* Better accessibility */
.button:focus-visible {
    box-shadow: 0 0 0 3px rgba(0,123,255,0.5);
}

:focus-within

/* Container with focused child */
form:focus-within {
    box-shadow: 0 0 10px rgba(0,123,255,0.3);
}

.search-container:focus-within {
    border-color: #007bff;
}

/* Highlight parent when input focused */
.form-group:focus-within label {
    color: #007bff;
    font-weight: 600;
}

🔗 Link Pseudo-classes

/* Unvisited link */
a:link {
    color: #007bff;
}

/* Visited link */
a:visited {
    color: #6610f2;
}

/* Order matters: LVHA (Link-Visited-Hover-Active) */
a:link { color: #007bff; }
a:visited { color: #6610f2; }
a:hover { color: #0056b3; }
a:active { color: #004085; }

/* Apply to all link states */
a:any-link {
    text-decoration: none;
}

📝 Form Pseudo-classes

:enabled & :disabled

/* Enabled input (default) */
input:enabled {
    background: white;
}

/* Disabled input */
input:disabled {
    background: #e9ecef;
    color: #6c757d;
    cursor: not-allowed;
}

button:disabled {
    opacity: 0.5;
    pointer-events: none;
}

:checked

/* Checked checkbox/radio */
input[type="checkbox"]:checked {
    background: #007bff;
}

/* Style label when checkbox checked */
input:checked + label {
    color: #007bff;
    font-weight: bold;
}

/* Toggle switch */
.switch input:checked ~ .slider {
    background: #4caf50;
}

/* Radio button checked */
input[type="radio"]:checked {
    border-color: #007bff;
}

:indeterminate

/* Checkbox in indeterminate state */
input[type="checkbox"]:indeterminate {
    background: #6c757d;
}

/* Progress bar without value */
progress:indeterminate {
    animation: progress-indeterminate 1.5s infinite;
}

:valid & :invalid

/* Valid input */
input:valid {
    border-color: #28a745;
}

/* Invalid input */
input:invalid {
    border-color: #dc3545;
}

/* Only show validation after interaction */
input:not(:placeholder-shown):valid {
    border-color: #28a745;
}

input:not(:placeholder-shown):invalid {
    border-color: #dc3545;
}

/* Email validation */
input[type="email"]:invalid {
    border-color: #dc3545;
}

input[type="email"]:valid {
    border-color: #28a745;
}

:required & :optional

/* Required field */
input:required {
    border-left: 3px solid #dc3545;
}

/* Optional field */
input:optional {
    border-left: 3px solid #6c757d;
}

/* Label for required field */
input:required + label::after {
    content: " *";
    color: red;
}

:read-only & :read-write

/* Read-only input */
input:read-only {
    background: #f8f9fa;
    cursor: default;
}

/* Editable input */
input:read-write {
    background: white;
}

/* contenteditable elements */
[contenteditable]:read-write {
    border: 1px solid #ced4da;
}

:placeholder-shown

/* When placeholder is visible */
input:placeholder-shown {
    border-color: #ced4da;
}

/* Hide label when placeholder shown */
input:placeholder-shown + label {
    opacity: 0;
}

/* Show label when typing */
input:not(:placeholder-shown) + label {
    opacity: 1;
    transform: translateY(-20px);
}

:in-range & :out-of-range

/* Number input within range */
input[type="number"]:in-range {
    border-color: #28a745;
}

/* Number input outside range */
input[type="number"]:out-of-range {
    border-color: #dc3545;
}

🔢 Structural Pseudo-classes

:first-child & :last-child

/* First child */
li:first-child {
    font-weight: bold;
}

p:first-child {
    margin-top: 0;
}

/* Last child */
li:last-child {
    border-bottom: none;
}

p:last-child {
    margin-bottom: 0;
}

:first-of-type & :last-of-type

/* First paragraph */
p:first-of-type {
    font-size: 1.2em;
}

/* Last image */
img:last-of-type {
    margin-bottom: 0;
}

/* Different from :first-child */
/* :first-of-type finds first of element type */

:nth-child()

/* Specific child */
li:nth-child(3) {
    color: red;  /* 3rd li */
}

/* Even children */
tr:nth-child(even) {
    background: #f8f9fa;
}

/* Odd children */
tr:nth-child(odd) {
    background: white;
}

/* Every 3rd element */
li:nth-child(3n) {
    color: blue;
}

/* Every 3rd starting from 2nd */
li:nth-child(3n+2) {
    color: green;
}

/* First 3 elements */
li:nth-child(-n+3) {
    font-weight: bold;
}

/* All except first 3 */
li:nth-child(n+4) {
    opacity: 0.7;
}

:nth-last-child()

/* Count from end */
li:nth-last-child(2) {
    color: red;  /* 2nd from end */
}

/* Last 3 items */
li:nth-last-child(-n+3) {
    font-weight: bold;
}

:nth-of-type()

/* Every 2nd paragraph */
p:nth-of-type(2n) {
    background: #f8f9fa;
}

/* First 3 images */
img:nth-of-type(-n+3) {
    border: 2px solid #007bff;
}

:only-child

/* Only child of parent */
li:only-child {
    list-style: none;
}

p:only-child {
    margin: 0;
}

:only-of-type

/* Only paragraph in container */
p:only-of-type {
    text-align: center;
}

/* Only image */
img:only-of-type {
    display: block;
    margin: 0 auto;
}

:empty

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

p:empty::before {
    content: "No content available";
    color: #6c757d;
}

/* Hide empty table cells */
td:empty {
    background: #f8f9fa;
}

🎯 Negation & Logical Pseudo-classes

:not()

/* All except specific selector */
li:not(.active) {
    opacity: 0.7;
}

/* Not first child */
p:not(:first-child) {
    margin-top: 1em;
}

/* Not disabled */
button:not(:disabled) {
    cursor: pointer;
}

/* Multiple not */
input:not([type="submit"]):not([type="button"]) {
    width: 100%;
}

/* Complex not */
:not(p):not(h1):not(h2) {
    color: #6c757d;
}

:is()

/* Match any of the selectors */
:is(h1, h2, h3) {
    line-height: 1.2;
}

/* Same as: */
h1, h2, h3 {
    line-height: 1.2;
}

/* More complex */
:is(.card, .panel) :is(h1, h2) {
    margin-top: 0;
}

/* Forgiving selector list */
:is(h1, :invalid-selector, h2) {
    /* Still works, ignores invalid */
}

:where()

/* Like :is() but with 0 specificity */
:where(h1, h2, h3) {
    margin: 0;
}

/* Easy to override */
h2 {
    margin-top: 1em;  /* This wins */
}

/* Useful for resets */
:where(ul, ol) {
    padding-left: 0;
}

:has()

/* Parent selector (relatively new) */
/* Card with image */
.card:has(img) {
    display: flex;
}

/* Form with error */
form:has(input:invalid) {
    border-color: #dc3545;
}

/* Li with nested ul */
li:has(> ul) {
    font-weight: bold;
}

/* Article with h2 */
article:has(h2) {
    padding-top: 2em;
}

🎯 Practical Examples

/* Striped table */
tr:nth-child(even) {
    background: #f8f9fa;
}

/* Highlight row on hover */
tr:hover {
    background: #e9ecef;
}

/* Form validation feedback */
input:focus {
    border-color: #007bff;
    outline: none;
}

input:valid {
    border-color: #28a745;
}

input:invalid:not(:placeholder-shown) {
    border-color: #dc3545;
}

/* Navigation active state */
nav a {
    color: #6c757d;
    transition: color 0.3s;
}

nav a:hover {
    color: #007bff;
}

nav a.active {
    color: #007bff;
    font-weight: bold;
}

/* Card interactions */
.card {
    transition: transform 0.3s, box-shadow 0.3s;
}

.card:hover {
    transform: translateY(-5px);
    box-shadow: 0 10px 20px rgba(0,0,0,0.1);
}

.card:active {
    transform: translateY(-2px);
}

/* Custom checkbox */
input[type="checkbox"] {
    display: none;
}

input[type="checkbox"] + label {
    position: relative;
    padding-left: 30px;
    cursor: pointer;
}

input[type="checkbox"] + label::before {
    content: "";
    position: absolute;
    left: 0;
    width: 20px;
    height: 20px;
    border: 2px solid #007bff;
    border-radius: 3px;
}

input[type="checkbox"]:checked + label::after {
    content: "✓";
    position: absolute;
    left: 4px;
    top: 0;
    color: #007bff;
}

/* Accordion */
.accordion-item {
    border-bottom: 1px solid #dee2e6;
}

.accordion-header {
    cursor: pointer;
    padding: 1rem;
}

.accordion-header:hover {
    background: #f8f9fa;
}

.accordion-content {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.3s;
}

.accordion-item.active .accordion-content {
    max-height: 500px;
}

/* Tabs */
.tab-button {
    padding: 0.5rem 1rem;
    border: none;
    background: transparent;
    cursor: pointer;
}

.tab-button:hover {
    background: #f8f9fa;
}

.tab-button.active {
    border-bottom: 2px solid #007bff;
    color: #007bff;
}

/* Dropdown menu */
.dropdown {
    position: relative;
}

.dropdown-menu {
    display: none;
    position: absolute;
    top: 100%;
    left: 0;
}

.dropdown:hover .dropdown-menu {
    display: block;
}

/* Focus styles for accessibility */
button:focus-visible {
    outline: 2px solid #007bff;
    outline-offset: 2px;
}

a:focus-visible {
    outline: 2px dashed #007bff;
    outline-offset: 3px;
}

/* Empty state */
.list:empty::before {
    content: "No items to display";
    display: block;
    padding: 2rem;
    text-align: center;
    color: #6c757d;
}

/* First and last spacing */
.section > *:first-child {
    margin-top: 0;
}

.section > *:last-child {
    margin-bottom: 0;
}

/* Alternate colors */
.tag:nth-child(4n+1) { background: #007bff; }
.tag:nth-child(4n+2) { background: #28a745; }
.tag:nth-child(4n+3) { background: #ffc107; }
.tag:nth-child(4n+4) { background: #dc3545; }

🎯 Key Takeaways

:hover/:active/:focus: User interaction states
:focus-visible: Only keyboard focus (better UX)
:nth-child(): Select elements by position (even, odd, formulas)
:not(): Exclude elements from selection
:checked/:disabled: Form element states
:valid/:invalid: Form validation styling
:has(): Parent selector (check if contains element)
Specificity: Pseudo-classes count as class selectors