2026年のモダンCSS：スタイルの書き方を変えた新機能たち

/* The old way: coupling component styles to page layout */
.card {
  display: flex;
  flex-direction: column;
}
 
@media (min-width: 768px) {
  .sidebar .card {
    /* Still vertical in sidebar */
  }
 
  .main-content .card {
    flex-direction: row;
    /* Horizontal in main area */
  }
}
 
@media (min-width: 1200px) {
  .hero .card {
    /* Yet another layout */
  }
}

/* The container query way: component knows only about itself */
.card-wrapper {
  container-type: inline-size;
  container-name: card;
}
 
.card {
  display: grid;
  grid-template-columns: 1fr;
  gap: 1rem;
}
 
@container card (min-width: 400px) {
  .card {
    grid-template-columns: 200px 1fr;
  }
}
 
@container card (min-width: 700px) {
  .card {
    grid-template-columns: 300px 1fr;
    gap: 2rem;
    font-size: 1.125rem;
  }
}

/* This is what you want 99% of the time */
.wrapper {
  container-type: inline-size;
}
 
/* This requires explicit height — rarely what you want */
.wrapper-both {
  container-type: size;
  height: 500px; /* required, or it collapses */
}

.page-layout {
  container-type: inline-size;
  container-name: layout;
}
 
.sidebar {
  container-type: inline-size;
  container-name: sidebar;
}
 
/* Target the sidebar specifically, not the nearest ancestor */
@container sidebar (max-width: 250px) {
  .nav-item {
    font-size: 0.875rem;
    padding: 0.25rem;
  }
}
 
/* Target the page layout */
@container layout (min-width: 1200px) {
  .page-header {
    font-size: 2.5rem;
  }
}

@container (min-width: 400px) {
  .card-title {
    font-size: clamp(1rem, 4cqi, 2rem);
  }
}

/* Declare layer order — this single line controls everything */
@layer reset, base, components, utilities, overrides;
 
@layer reset {
  *,
  *::before,
  *::after {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
  }
}
 
@layer base {
  body {
    font-family: system-ui, sans-serif;
    line-height: 1.6;
    color: oklch(20% 0 0);
  }
 
  h1, h2, h3 {
    line-height: 1.2;
    text-wrap: balance;
  }
}
 
@layer components {
  .card {
    background: white;
    border-radius: 0.5rem;
    padding: 1.5rem;
    box-shadow: 0 1px 3px oklch(0% 0 0 / 0.12);
  }
 
  .card .title {
    font-size: 1.25rem;
    font-weight: 600;
    color: oklch(25% 0.05 260);
  }
}
 
@layer utilities {
  .text-red {
    color: oklch(55% 0.25 25);
  }
 
  .hidden {
    display: none;
  }
 
  .sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
  }
}

@layer theme, base, components, utilities;

@layer reset, tokens, base, layouts, components, utilities, overrides;

@layer components {
  .modal {
    background: white;
  }
}
 
/* Not in any layer — wins over everything in layers */
.special-page .modal {
  background: oklch(95% 0.02 260);
}

@layer third-party {
  @import url("some-library.css");
}

/* Style a form group when its input has focus */
.form-group:has(input:focus) {
  border-color: oklch(55% 0.2 260);
  box-shadow: 0 0 0 3px oklch(55% 0.2 260 / 0.15);
}
 
/* Style a card differently when it contains an image */
.card:has(img) {
  padding-top: 0;
}
 
.card:has(img) .card-content {
  padding: 1.5rem;
}
 
/* A card without an image gets different treatment */
.card:not(:has(img)) {
  border-left: 4px solid oklch(55% 0.2 260);
}

/* The field wrapper reacts to its input's validity */
.field:has(input:invalid:not(:placeholder-shown)) {
  --field-color: oklch(55% 0.25 25);
}
 
.field:has(input:valid:not(:placeholder-shown)) {
  --field-color: oklch(55% 0.2 145);
}
 
.field {
  --field-color: oklch(70% 0 0);
  border: 2px solid var(--field-color);
  border-radius: 0.5rem;
  padding: 0.75rem;
  transition: border-color 0.2s;
}
 
.field label {
  color: var(--field-color);
  font-size: 0.875rem;
  font-weight: 500;
}
 
.field .error-message {
  display: none;
  color: oklch(55% 0.25 25);
  font-size: 0.8rem;
  margin-top: 0.25rem;
}
 
.field:has(input:invalid:not(:placeholder-shown)) .error-message {
  display: block;
}

/* Adjust grid columns based on how many items exist */
.grid-auto:has(> :nth-child(4)) {
  grid-template-columns: repeat(2, 1fr);
}
 
.grid-auto:has(> :nth-child(7)) {
  grid-template-columns: repeat(3, 1fr);
}
 
.grid-auto:has(> :nth-child(13)) {
  grid-template-columns: repeat(4, 1fr);
}

.page-layout {
  display: grid;
  grid-template-columns: 1fr;
  gap: 2rem;
}
 
/* If the layout contains a sidebar, switch to two columns */
.page-layout:has(.sidebar) {
  grid-template-columns: 1fr 300px;
}
 
/* Adjust main content width when sidebar is present */
.page-layout:has(.sidebar) .main-content {
  max-width: 65ch;
}

/* Style an article only when it has a specific class of figure */
article:has(figure.full-bleed) {
  overflow: visible;
}
 
/* A navigation that changes when it contains a search input */
nav:has(input[type="search"]:focus) {
  background: oklch(98% 0 0);
  box-shadow: 0 2px 8px oklch(0% 0 0 / 0.08);
}
 
/* Enable dark-mode at the component level */
.theme-switch:has(input:checked) ~ main {
  color-scheme: dark;
  background: oklch(15% 0 0);
  color: oklch(90% 0 0);
}

.card {
  background: white;
  border-radius: 0.5rem;
  padding: 1.5rem;
 
  .title {
    font-size: 1.25rem;
    font-weight: 600;
    margin-bottom: 0.5rem;
  }
 
  .description {
    color: oklch(40% 0 0);
    line-height: 1.7;
  }
 
  &:hover {
    box-shadow: 0 4px 12px oklch(0% 0 0 / 0.1);
  }
 
  &.featured {
    border: 2px solid oklch(60% 0.2 260);
  }
}

/* CSS Nesting — works now in all browsers */
.parent {
  /* Direct class/element nesting works without & */
  .child {
    color: red;
  }
 
  /* & is required for pseudo-classes and compound selectors */
  &:hover {
    background: blue;
  }
 
  &.active {
    font-weight: bold;
  }
 
  /* Nested media queries — this is great */
  @media (width >= 768px) {
    flex-direction: row;
  }
}

.hero {
  padding: 2rem;
  font-size: 1rem;
 
  @media (width >= 768px) {
    padding: 4rem;
    font-size: 1.25rem;
  }
 
  @media (width >= 1200px) {
    padding: 6rem;
    font-size: 1.5rem;
  }
}

/* Good — two levels */
.nav {
  .link {
    color: inherit;
 
    &:hover {
      color: oklch(55% 0.2 260);
    }
  }
}
 
/* Bad — specificity nightmare */
.page {
  .layout {
    .sidebar {
      .nav {
        .list {
          .item {
            .link {
              color: red; /* good luck overriding this */
            }
          }
        }
      }
    }
  }
}

:root {
  /* oklch(lightness chroma hue) */
  --color-primary: oklch(55% 0.2 260);     /* blue */
  --color-secondary: oklch(55% 0.2 330);   /* purple */
  --color-success: oklch(55% 0.2 145);     /* green */
  --color-danger: oklch(55% 0.25 25);      /* red */
  --color-warning: oklch(75% 0.18 85);     /* yellow — higher L for equal perception */
 
  /* These all look equally "medium" to the human eye */
  /* With HSL, you'd need different lightness values for each hue */
}

:root {
  --brand: oklch(55% 0.2 260);
 
  /* Lighten by mixing with white */
  --brand-light: color-mix(in oklch, var(--brand) 30%, white);
 
  /* Darken by mixing with black */
  --brand-dark: color-mix(in oklch, var(--brand) 70%, black);
 
  /* Create a subtle background */
  --brand-bg: color-mix(in oklch, var(--brand) 8%, white);
 
  /* Semi-transparent version */
  --brand-overlay: color-mix(in oklch, var(--brand) 50%, transparent);
}
 
.button {
  background: var(--brand);
  color: white;
 
  &:hover {
    background: var(--brand-dark);
  }
 
  &:active {
    background: color-mix(in oklch, var(--brand) 60%, black);
  }
}
 
.button.secondary {
  background: var(--brand-bg);
  color: var(--brand-dark);
 
  &:hover {
    background: color-mix(in oklch, var(--brand) 15%, white);
  }
}

:root {
  --hue: 260;
  --chroma: 0.2;
 
  --color-50:  oklch(97% calc(var(--chroma) * 0.1) var(--hue));
  --color-100: oklch(93% calc(var(--chroma) * 0.2) var(--hue));
  --color-200: oklch(85% calc(var(--chroma) * 0.4) var(--hue));
  --color-300: oklch(75% calc(var(--chroma) * 0.6) var(--hue));
  --color-400: oklch(65% calc(var(--chroma) * 0.8) var(--hue));
  --color-500: oklch(55% var(--chroma) var(--hue));
  --color-600: oklch(48% var(--chroma) var(--hue));
  --color-700: oklch(40% calc(var(--chroma) * 0.9) var(--hue));
  --color-800: oklch(32% calc(var(--chroma) * 0.8) var(--hue));
  --color-900: oklch(25% calc(var(--chroma) * 0.7) var(--hue));
  --color-950: oklch(18% calc(var(--chroma) * 0.5) var(--hue));
}

.progress-bar {
  position: fixed;
  top: 0;
  left: 0;
  height: 3px;
  background: oklch(55% 0.2 260);
  transform-origin: left;
  z-index: 1000;
 
  animation: grow-progress linear both;
  animation-timeline: scroll();
}
 
@keyframes grow-progress {
  from {
    transform: scaleX(0);
  }
  to {
    transform: scaleX(1);
  }
}

.reveal {
  animation: reveal linear both;
  animation-timeline: view();
  animation-range: entry 0% entry 100%;
}
 
@keyframes reveal {
  from {
    opacity: 0;
    transform: translateY(30px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.parallax-bg {
  animation: parallax linear both;
  animation-timeline: scroll();
}
 
@keyframes parallax {
  from {
    transform: translateY(-20%);
  }
  to {
    transform: translateY(20%);
  }
}
 
.parallax-section {
  overflow: hidden;
  position: relative;
}

.scroller {
  overflow-y: auto;
  scroll-timeline-name: --my-scroller;
  scroll-timeline-axis: block;
}
 
/* An element anywhere in the DOM can reference this timeline */
.indicator {
  animation: progress linear both;
  animation-timeline: --my-scroller;
}
 
@keyframes progress {
  from { width: 0%; }
  to { width: 100%; }
}

.trigger {
  anchor-name: --my-trigger;
}
 
.tooltip {
  position: fixed;
  position-anchor: --my-trigger;
 
  /* Position the tooltip's top-center at the trigger's bottom-center */
  top: anchor(bottom);
  left: anchor(center);
  translate: -50% 0;
 
  /* Fallback positioning if it overflows the viewport */
  position-try-fallbacks: flip-block, flip-inline;
}

.dropdown-menu {
  position: fixed;
  position-anchor: --menu-button;
 
  /* Default: below the button, aligned to the left edge */
  top: anchor(bottom);
  left: anchor(left);
 
  /* If it doesn't fit below, try above. If it doesn't fit left-aligned, try right-aligned */
  position-try-fallbacks: flip-block, flip-inline;
 
  /* Add a gap between the anchor and the dropdown */
  margin-top: 4px;
}

@position-try --above-right {
  bottom: anchor(top);
  right: anchor(right);
  top: auto;
  left: auto;
  margin-top: 0;
  margin-bottom: 4px;
}
 
@position-try --left-center {
  right: anchor(left);
  top: anchor(center);
  left: auto;
  bottom: auto;
  translate: 0 -50%;
  margin-top: 0;
  margin-right: 4px;
}
 
.tooltip {
  position: fixed;
  position-anchor: --trigger;
  top: anchor(bottom);
  left: anchor(center);
  translate: -50% 0;
  margin-top: 4px;
 
  position-try-fallbacks: --above-right, --left-center;
}

<button popovertarget="my-popover" style="anchor-name: --btn">Settings</button>
 
<div popover id="my-popover" style="
  position-anchor: --btn;
  top: anchor(bottom);
  left: anchor(center);
  translate: -50% 0;
  margin-top: 8px;
">
  Popover content here
</div>

/* Parent grid */
.card-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 1.5rem;
}
 
/* Each card becomes a subgrid, aligning rows to the parent */
.card {
  display: grid;
  grid-row: span 3; /* Card spans 3 rows in the parent */
  grid-template-rows: subgrid;
  gap: 0; /* Card controls its own internal gap */
}
 
.card-title {
  font-weight: 600;
  padding: 1rem 1rem 0.5rem;
}
 
.card-body {
  padding: 0 1rem;
  color: oklch(40% 0 0);
}
 
.card-footer {
  padding: 0.5rem 1rem 1rem;
  margin-top: auto;
  border-top: 1px solid oklch(90% 0 0);
}

/* The old hack — fragile and breaks with dynamic content */
.card-title {
  min-height: 3rem; /* pray that no title exceeds this */
}
 
.card-body {
  min-height: 8rem; /* even more prayer */
}

.form {
  display: grid;
  grid-template-columns: auto 1fr;
  gap: 0.75rem 1rem;
}
 
.form-row {
  display: grid;
  grid-column: 1 / -1;
  grid-template-columns: subgrid;
  align-items: center;
}
 
.form-row label {
  grid-column: 1;
  text-align: right;
  font-weight: 500;
}
 
.form-row input {
  grid-column: 2;
  padding: 0.5rem;
  border: 1px solid oklch(80% 0 0);
  border-radius: 0.375rem;
}

@view-transition {
  navigation: auto;
}

/* On both pages */
.page-header {
  view-transition-name: header;
}
 
.main-content {
  view-transition-name: content;
}
 
.hero-image {
  view-transition-name: hero;
}
 
/* Transition animations */
::view-transition-old(content) {
  animation: slide-out 0.3s ease-in both;
}
 
::view-transition-new(content) {
  animation: slide-in 0.3s ease-out both;
}
 
::view-transition-old(hero) {
  animation: fade-out 0.2s ease-in both;
}
 
::view-transition-new(hero) {
  animation: none; /* The new hero just appears */
}
 
@keyframes slide-out {
  to {
    transform: translateX(-100%);
    opacity: 0;
  }
}
 
@keyframes slide-in {
  from {
    transform: translateX(100%);
    opacity: 0;
  }
}
 
@keyframes fade-out {
  to {
    opacity: 0;
  }
}

/* CSS side — define your transition animations */
::view-transition-old(root) {
  animation: fade-out 0.2s ease-in;
}
 
::view-transition-new(root) {
  animation: fade-in 0.2s ease-out;
}

// JS side — wrap your DOM update in startViewTransition
document.startViewTransition(() => {
  // Update the DOM here — React render, innerHTML swap, etc.
  updateContent(newPageData);
});

@media (prefers-reduced-motion: reduce) {
  ::view-transition-group(*),
  ::view-transition-old(*),
  ::view-transition-new(*) {
    animation: none !important;
  }
}

/* Before: Sass */
$primary: #3b82f6;
 
.card {
  background: white;
  border: 1px solid lighten($primary, 40%);
 
  &:hover {
    border-color: $primary;
  }
 
  .title {
    color: darken($primary, 15%);
  }
}
 
/* After: Native CSS */
.card {
  --primary: oklch(55% 0.2 260);
 
  background: white;
  border: 1px solid color-mix(in oklch, var(--primary) 20%, white);
 
  &:hover {
    border-color: var(--primary);
  }
 
  .title {
    color: color-mix(in oklch, var(--primary) 70%, black);
  }
}

// Before: JavaScript you can delete
const observer = new IntersectionObserver(
  (entries) => {
    entries.forEach((entry) => {
      if (entry.isIntersecting) {
        entry.target.classList.add('visible');
        observer.unobserve(entry.target);
      }
    });
  },
  { threshold: 0.1 }
);
 
document.querySelectorAll('.reveal').forEach((el) => observer.observe(el));

/* After: CSS that replaces all of the above */
.reveal {
  animation: reveal linear both;
  animation-timeline: view();
  animation-range: entry 0% entry 100%;
}
 
@keyframes reveal {
  from { opacity: 0; transform: translateY(20px); }
  to { opacity: 1; transform: translateY(0); }
}

/* Old way */
.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 1rem;
}
 
/* Better: CSS Grid or Flexbox */
.page {
  display: grid;
  place-items: center;
}
 
/* Or: container queries make the container itself responsive */
.content {
  container-type: inline-size;
  width: min(100% - 2rem, 1200px);
  margin-inline: auto; /* OK, still auto, but margin-inline is clearer */
}

/* Before: Media query for component styling (wrong scope) */
@media (min-width: 768px) {
  .product-card {
    flex-direction: row;
  }
}
 
/* After: Container query (right scope) */
.product-card-wrapper {
  container-type: inline-size;
}
 
@container (min-width: 400px) {
  .product-card {
    flex-direction: row;
  }
}

@layer components {
  .card-wrapper {
    container-type: inline-size;
  }
 
  .card {
    --accent: oklch(55% 0.2 260);
 
    display: grid;
    gap: 1rem;
    padding: 1.5rem;
    border-radius: 0.5rem;
    background: white;
    border: 1px solid oklch(90% 0 0);
 
    &:has(img) {
      padding-top: 0;
 
      img {
        border-radius: 0.5rem 0.5rem 0 0;
        width: 100%;
        aspect-ratio: 16 / 9;
        object-fit: cover;
      }
    }
 
    &:has(.badge.new) {
      border-color: var(--accent);
    }
 
    .title {
      font-size: 1.125rem;
      font-weight: 600;
      color: oklch(20% 0 0);
      text-wrap: balance;
    }
 
    .meta {
      font-size: 0.875rem;
      color: oklch(50% 0 0);
    }
 
    @container (min-width: 500px) {
      grid-template-columns: 200px 1fr;
 
      &:has(img) {
        padding: 0;
 
        img {
          border-radius: 0.5rem 0 0 0.5rem;
          height: 100%;
          aspect-ratio: auto;
        }
      }
 
      .content {
        padding: 1.5rem;
      }
    }
  }
}