Lesson 09 · Advanced CSS: pseudo-classes, transitions, and animations¶

What you will learn

Pseudo-classes (:hover, :focus, :nth-child) and pseudo-elements (::before, ::after)
Transitions smoothly between two states
Transform: rotation, scaling, translation
@keyframes — full animations
Popular modern effects you use on any site

Pseudo-classes¶

A pseudo-class attaches to a selector with : and selects the element in a particular state.

Interaction pseudo-classes¶

/* The mouse is over the element */
.button:hover {
  background-color: #4338ca;
}

/* The element is focused (tab in a form) */
input:focus {
  border-color: #4f46e5;
  outline: 2px solid #c7d2fe;
}

/* At the moment of the click (pressed) */
.button:active {
  transform: translateY(1px);
}

/* Already visited link */
a:visited {
  color: #7c3aed;
}

Don't remove :focus visually

outline: none without putting something else in its place breaks accessibility. Keyboard users (including those with disabilities) no longer know where they are. Always leave a visible border at :focus.

Structural pseudo-classes¶

/* The first child of the parent */
li:first-child {
  font-weight: 700;
}

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

/* Even rows in a table */
tr:nth-child(even) {
  background-color: #f9fafb;
}

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

/* Every third element */
.card:nth-child(3n) {
  background-color: #ede9fe;
}

/* All, EXCEPT .active */
.tab:not(.active) {
  opacity: 0.6;
}

Pseudo-classes for forms¶

input:disabled {
  background-color: #f3f4f6;
  cursor: not-allowed;
}

input:checked + label {
  font-weight: 700;
}

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

input:valid {
  border-color: green;
}

input:invalid {
  border-color: red;
}

Pseudo-elements¶

Two colons (::) instead of one. Creates virtual content that doesn't exist in the HTML.

.quote::before {
  content: "“";
  color: #9ca3af;
  font-size: 2rem;
}

.quote::after {
  content: "”";
  color: #9ca3af;
  font-size: 2rem;
}

Common uses:

Decorative quotation marks
Icons before a link
Separator between links (::after { content: " · "; })
Overlays over images

/* Breadcrumb with automatic separator */
.breadcrumb li + li::before {
  content: " › ";
  color: #9ca3af;
  padding: 0 0.5rem;
}

Transitions¶

A transition animates the change of a property from one value to another.

Syntax¶

.button {
  background-color: #4f46e5;
  transition: background-color 0.3s ease;
  /*          ^prop          ^duration ^curve */
}

.button:hover {
  background-color: #4338ca;   /* smoothly transitions from one color to another */
}

Multiple properties¶

.card {
  transition:
    transform 0.3s ease,
    box-shadow 0.3s ease;
}

`all` — all properties¶

.button {
  transition: all 0.2s ease;   /* convenient, but less performant */
}

Timing functions (animation curves)¶

Value	Character
`linear`	constant speed (robotic)
`ease` (default)	starts slow, accelerates, slows down
`ease-in`	starts slow, ends fast
`ease-out`	starts fast, ends smooth
`ease-in-out`	slow at the ends, fast in the middle
`cubic-bezier(...)`	custom curve

ease-out for UI

Effects that appear to the user (hover, fade-in) feel most natural with ease-out — they move quickly, then slow down.

Transform — rotation, scaling, translation¶

transform applies geometric transformations without affecting the layout. Other elements don't move when an element is transformed.

.element {
  /* Translation — movement on X / Y */
  transform: translate(10px, -5px);
  transform: translateX(20px);
  transform: translateY(-10px);

  /* Scaling */
  transform: scale(1.1);         /* 110% */
  transform: scaleX(2);          /* 2 times wider */

  /* Rotation */
  transform: rotate(45deg);
  transform: rotate(-90deg);

  /* Skew */
  transform: skew(10deg, 0);

  /* Combined — applied right to left */
  transform: translateY(-4px) scale(1.05) rotate(2deg);
}

Why transform is so powerful

The browser runs transform and opacity on the GPU, using compositing. Animations are smooth even on old phones — unlike margin or top, which trigger layout reflow (slow).

Animations with `@keyframes`¶

For complex animations (multiple steps) you use @keyframes + animation.

Syntax¶

@keyframes pulse {
  0%   { transform: scale(1); }
  50%  { transform: scale(1.1); }
  100% { transform: scale(1); }
}

.pulsing {
  animation: pulse 2s ease-in-out infinite;
  /*         ^name ^duration ^curve ^repeat */
}

Detailed syntax¶

.element {
  animation-name: pulse;
  animation-duration: 2s;
  animation-timing-function: ease-in-out;
  animation-delay: 0s;
  animation-iteration-count: infinite;   /* or a number: 3, 1, ... */
  animation-direction: normal;           /* normal | reverse | alternate */
  animation-fill-mode: forwards;         /* keeps the final state */
}

Examples¶

/* Fade-in on page load */
@keyframes fadeIn {
  from { opacity: 0; transform: translateY(10px); }
  to   { opacity: 1; transform: translateY(0); }
}

.hero {
  animation: fadeIn 0.6s ease-out;
}

/* Spinner */
@keyframes spin {
  to { transform: rotate(360deg); }
}

.loader {
  animation: spin 1s linear infinite;
}

/* Shake (for form errors) */
@keyframes shake {
  0%, 100% { transform: translateX(0); }
  25%      { transform: translateX(-8px); }
  75%      { transform: translateX(8px); }
}

.input-error {
  animation: shake 0.3s ease;
}

Respect prefers-reduced-motion

Some users disable animations for medical reasons (motion sickness, epilepsy). Provide a simplified variant:

@media (prefers-reduced-motion: reduce) {
  * {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}

Popular effects¶

Button with "lift" on hover¶

.button {
  background: #4f46e5;
  color: white;
  padding: 0.75rem 1.5rem;
  border: none;
  border-radius: 8px;
  cursor: pointer;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.button:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 16px rgba(79, 70, 229, 0.3);
}

Card that grows on hover¶

.card {
  transition: transform 0.3s ease;
}

.card:hover {
  transform: scale(1.03);
}

Animated underline¶

.link {
  position: relative;
  text-decoration: none;
  color: #1f2937;
}

.link::after {
  content: "";
  position: absolute;
  left: 0;
  bottom: -2px;
  width: 100%;
  height: 2px;
  background: #4f46e5;
  transform: scaleX(0);
  transform-origin: left;
  transition: transform 0.3s ease;
}

.link:hover::after {
  transform: scaleX(1);
}

Exercises¶

Exercise 1 — Button with colored hover¶

The button changes its background color smoothly (0.3s) on hover.

Solution

<button class="cta">Subscribe</button>

.cta {
  background-color: #10b981;
  color: white;
  padding: 0.75rem 1.5rem;
  border: none;
  border-radius: 8px;
  font-size: 1rem;
  cursor: pointer;
  transition: background-color 0.3s ease;
}

.cta:hover {
  background-color: #059669;
}

Exercise 2 — First element with different style¶

The first <li> in a list has a different color.

Solution

li:first-child {
  color: #4f46e5;
  font-weight: 700;
}

Exercise 3 — Even rows in a table¶

The even rows have a light gray background (zebra effect).

Solution

tr:nth-child(even) {
  background-color: #f9fafb;
}

tr:hover {
  background-color: #e0e7ff;
}

Exercise 4 — Beating heart¶

A heart that pulses in a loop, like a heartbeat.

Solution

<div class="heart">&#9829;</div>

.heart {
  font-size: 4rem;
  color: #e11d48;
  display: inline-block;
  animation: heartbeat 1.2s ease-in-out infinite;
}

@keyframes heartbeat {
  0%, 100% { transform: scale(1); }
  25%      { transform: scale(1.2); }
  50%      { transform: scale(1); }
  75%      { transform: scale(1.15); }
}

/* Respect user preferences */
@media (prefers-reduced-motion: reduce) {
  .heart { animation: none; }
}

Mini-project: Modern "premium" button¶

A button with hover: lift, shadow, gradient shift, and smooth transition.

Solution

index.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Premium button</title>
  <link rel="stylesheet" href="styles.css">
</head>
<body>
  <main>
    <button class="premium-button">
      Get started
    </button>
  </main>
</body>
</html>

styles.css

* { box-sizing: border-box; margin: 0; padding: 0; }

body {
  font-family: "Inter", sans-serif;
  background: #f9fafb;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
}

.premium-button {
  position: relative;
  padding: 1rem 2.5rem;
  font-size: 1.1rem;
  font-weight: 600;
  color: white;
  border: none;
  border-radius: 12px;
  cursor: pointer;

  /* Background gradient */
  background: linear-gradient(135deg, #6366f1, #8b5cf6);
  background-size: 200% 200%;
  background-position: 0% 50%;

  /* Initial shadow */
  box-shadow: 0 4px 14px rgba(99, 102, 241, 0.3);

  /* Transition covers all properties changed on hover */
  transition:
    transform 0.25s ease,
    box-shadow 0.25s ease,
    background-position 0.6s ease;
}

.premium-button:hover {
  /* Lift effect */
  transform: translateY(-3px);
  /* Larger and more diffuse shadow */
  box-shadow: 0 10px 28px rgba(99, 102, 241, 0.45);
  /* The gradient shifts subtly */
  background-position: 100% 50%;
}

.premium-button:active {
  /* Slight press on click */
  transform: translateY(-1px);
}

/* Visible focus for accessibility */
.premium-button:focus-visible {
  outline: 3px solid #c7d2fe;
  outline-offset: 2px;
}

/* Respect user preferences */
@media (prefers-reduced-motion: reduce) {
  .premium-button { transition: none; }
  .premium-button:hover { transform: none; }
}

How it works:

background-size: 200% makes the gradient larger than the button
background-position shifts the "window" that shows part of the gradient
On hover, the position changes from 0% to 100% → color flow effect
translateY(-3px) + larger shadow → impression of floating
:focus-visible ensures accessibility for keyboard users

Summary¶

Pseudo-classes (:hover, :focus, :nth-child) select the element in a state
Pseudo-elements (::before, ::after) create decorative content without HTML
transition animates changes between states — minimum for modern UI
transform (translate, scale, rotate) is fast — runs on the GPU
@keyframes + animation for animations with multiple steps
Respect prefers-reduced-motion for accessibility
Keep a visual indicator at :focus

Next step: → Lesson 10: Responsive design

Lesson 09 · Advanced CSS: pseudo-classes, transitions, and animations¶

Pseudo-classes¶

Interaction pseudo-classes¶

Structural pseudo-classes¶

Pseudo-classes for forms¶

Pseudo-elements¶

Transitions¶

Syntax¶

Multiple properties¶

all — all properties¶

Timing functions (animation curves)¶

Transform — rotation, scaling, translation¶

Animations with @keyframes¶

Syntax¶

Detailed syntax¶

Examples¶

Popular effects¶

Button with "lift" on hover¶

Card that grows on hover¶

Animated underline¶

Exercises¶

Exercise 1 — Button with colored hover¶

Exercise 2 — First element with different style¶

Exercise 3 — Even rows in a table¶

Exercise 4 — Beating heart¶

Mini-project: Modern "premium" button¶

Summary¶

`all` — all properties¶

Animations with `@keyframes`¶