Modern CSS

.card {
  container: --my-card / inline-size;
}

@container --my-card (width < 40ch) {
  /* Component-based responsive design */
}

@container (20ch < width < 50ch) {
  /* Range syntax */
}

.tooltip {
  container-type: anchored;
}

@container anchored(top) {
  /* Styles when positioned at top */
}

@media (width <= 1024px) { }
@media (360px < width < 1024px) { }

:root {
  color-scheme: light dark;
  --surface-1: light-dark(white, #222);
  --text-1: light-dark(#222, #fff);
}

/* OKLCH: uniform brightness, P3+ colors */
.vibrant {
  background: oklch(72% 75% 330);
}

/* Display-P3 for HDR displays */
@media (dynamic-range: high) {
  .neon {
    --neon-red: color(display-p3 1 0 0);
  }
}

/* Better gradients with in oklch */
.gradient {
  background: linear-gradient(
    to right in oklch,
    color(display-p3 1 0 .5),
    color(display-p3 0 1 1)
  );
}

/* color-mix() */
.lighten {
  background: color-mix(in oklab, var(--brand), white);
}

/* Relative color syntax */
.lighter {
  background: oklch(from blue calc(l + .25) c h);
  background: oklch(from blue 75% c h); /* Set to specific lightness */
}

.semi-transparent {
  background: oklch(from var(--color) l c h / 50%);
}

.complementary {
  background: hsl(from blue calc(h + 180) s l);
}

:root {
  accent-color: hotpink; /* Tints checkboxes, radios, range inputs */
}

h1 {
  text-wrap: balance; /* Balanced multi-line headings */
  max-inline-size: 25ch;
}

p {
  text-wrap: pretty; /* No orphans */
  max-inline-size: 50ch;
}

h1, p, button {
  text-box: trim-both cap alphabetic; /* Optical vertical centering */
}

.heading {
  font-size: clamp(1rem, 1rem + 0.5vw, 2rem); /* Respects user preferences */
}

/* Animate on scroll position */
.parallax {
  animation: slide-up linear both;
  animation-timeline: scroll();
}

/* Animate on viewport intersection */
.fade-in {
  animation: fade linear both;
  animation-timeline: view();
  animation-range: cover -75cqi contain 20cqi;
}

@view-transition {
  navigation: auto; /* Automatically animate page transitions (MPAs) */
}

nav {
  view-transition-name: --persist-nav; /* Persist specific elements */
  view-transition-class: --site-header; /* Group transitions with classes */
}

/* Style the active transition */
html:active-view-transition {
  overflow: hidden;
}

.springy {
  --spring: linear(
    0, 0.14 4%, 0.94 17%, 1.15 24% 30%, 1.02 43%, 0.98 51%, 1 77%, 1
  );
  transition: transform 1s var(--spring);
}

.dialog {
  transition: opacity .5s, scale .5s;

  @starting-style {
    opacity: 0;
    scale: 1.1;
  }
}

@property --gradient-angle {
  syntax: "<angle>";
  inherits: false;
  initial-value: 0deg;
}

.animate {
  transition: --gradient-angle 1s ease;

  &:hover {
    --gradient-angle: 360deg;
  }
}

/* Finally: Animate to auto height! */
.accordion-content {
  height: 0;
  overflow: hidden;
  transition: height 0.3s ease;
}

.accordion-item.open .accordion-content {
  height: calc-size(auto);
}

.radial-layout {
  --_angle: calc(var(--sibling-index) * var(--_offset));
  translate:
    calc(cos(var(--_angle)) * var(--_circle-size))
    calc(sin(var(--_angle)) * var(--_circle-size));
}

.staggered {
  animation-delay: calc(sibling-index() * .1s);
  background-color: hsl(sibling-count() 50% 50%);
}

.dynamic {
  color: if(
    style(--theme: dark),
    white,
    black
  );
}

@layer reset, design-system, components, utilities;

@import "open-props/colors" layer(design-system);
@import "components/nav/base.css" layer(components.nav);

@layer components.nav.primary {
  nav {
    position: sticky;
    inset-block-start: 0;
  }
}

<dialog id="modal">
  <form method="dialog">
    <button value="cancel">Cancel</button>
    <button value="confirm">Confirm</button>
  </form>
</dialog>

<button commandfor="modal" command="showModal">Open</button>
<button commandfor="modal" command="close">Close</button>

<button popovertarget="menu">Show Menu</button>
<div popover id="menu">...</div>

[popover] {
  transition:
    display .5s allow-discrete,
    overlay .5s allow-discrete,
    opacity .5s;

  @starting-style {
    &:popover-open {
      opacity: 0;
    }
  }
}

.tooltip-anchor {
  anchor-name: --tooltip;
}

.tooltip[popover] {
  position-anchor: --tooltip;
  position-area: block-start;
  position-try-fallbacks: flip-block;
  position-try-order: most-height;
}

<details name="accordion">...</details>
<details name="accordion">...</details>
<!-- Only one can be open at a time -->

select {
  appearance: base-select; /* Full CSS control */
}

/* Style options with rich HTML */
select option::before {
  content: ""; /* Can include images, icons */
}

<search>
  <form>
    <input type="search" name="q">
    <button type="submit">Search</button>
  </form>
</search>

textarea, select, input {
  field-sizing: content; /* Auto-grow to content */
}

textarea {
  min-block-size: 3lh; /* Line-height units */
  max-block-size: 80dvh;
}

/* Wait for user interaction before showing errors */
:user-invalid {
  outline-color: red;
}

:user-valid {
  outline-color: green;
}

label:has(+ input:user-invalid) {
  text-decoration: underline wavy red;
}

<select>
  <option>Option 1</option>
  <hr>
  <option>Option 2</option>
</select>

.custom-scrollbar {
  scrollbar-color: hotpink transparent;
  scrollbar-width: thin;
}

.complex-clip {
  clip-path: shape(
    from 0% 0%,
    curve by 50% 25% via 25% 50%,
    line to 100% 100%
  );
}

.fancy-corners {
  corner-shape: squircle;
  corner-shape: notch;
  corner-shape: scoop;
  corner-shape: superellipse(0.7);
}

@supports (animation-timeline: view()) {
  .fade-in {
    animation: fade linear both;
    animation-timeline: view();
  }
}

@supports (container-type: inline-size) {
  .responsive-card {
    container-type: inline-size;
  }
}

@media (prefers-reduced-motion: no-preference) {
  .animated {
    animation: slide 1s ease;
  }
}

@media (prefers-color-scheme: dark) {
  :root {
    --surface: #222;
  }
}

@media (prefers-contrast: more) {
  .text {
    font-weight: 600;
  }
}