:root {
  --color-bg: #FAFAFA;
  --color-card: #FFFFFF;
  --color-primary: #2563EB;
  --color-primary-hover: #1D4ED8;
  --color-primary-active: #1E40AF;
  --color-accent: #F59E0B;
  --color-text: #111827;
  --color-text-secondary: #6B7280;
  --color-text-tertiary: #9CA3AF;
  --color-border: #E5E7EB;
  --color-success: #10B981;
  --color-error: #EF4444;
  --color-overlay: rgba(0, 0, 0, 0.3);

  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;

  --size-hero: clamp(2.5rem, 5vw, 4rem);
  --size-section: clamp(1.5rem, 2.5vw, 1.875rem);
  --size-card-title: 1.125rem;
  --size-body: 1rem;
  --size-caption: 0.875rem;
  --size-small: 0.75rem;

  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 16px;
  --radius-full: 9999px;

  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.08), 0 2px 4px -2px rgba(0, 0, 0, 0.05);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.08), 0 4px 6px -4px rgba(0, 0, 0, 0.04);
  --shadow-xl: 0 20px 25px -5px rgba(0, 0, 0, 0.08), 0 8px 10px -6px rgba(0, 0, 0, 0.04);

  --header-height: 64px;
  --header-height-scrolled: 56px;

  --max-width: 1280px;
  --content-padding: 1.5rem;
}

@media (min-width: 768px) {
  :root {
    --content-padding: 2rem;
  }
}

@media (min-width: 1024px) {
  :root {
    --content-padding: 2.5rem;
  }
}

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

html {
  font-size: 16px;
  -webkit-text-size-adjust: 100%;
  scroll-behavior: smooth;
}

body {
  font-family: var(--font-sans);
  font-size: var(--size-body);
  line-height: 1.6;
  color: var(--color-text);
  background: var(--color-bg);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  min-height: 100vh;
}

img {
  max-width: 100%;
  height: auto;
  display: block;
}

a {
  color: inherit;
  text-decoration: none;
}

button {
  font: inherit;
  cursor: pointer;
  border: none;
  background: none;
}

input {
  font: inherit;
}

ul, ol {
  list-style: none;
}

h1, h2, h3, h4, h5, h6 {
  line-height: 1.2;
  font-weight: 700;
}

.skip-link {
  position: fixed;
  top: -100%;
  left: 1rem;
  z-index: 1000;
  padding: 0.5rem 1rem;
  background: var(--color-primary);
  color: #fff;
  border-radius: var(--radius-sm);
  font-size: var(--size-caption);
  font-weight: 600;
  transition: top 0.2s;
}

.skip-link:focus {
  top: 1rem;
}

.container {
  width: 100%;
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 var(--content-padding);
}

.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(250, 250, 250, 0.85);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid transparent;
  transition: height 0.3s ease, border-color 0.3s ease, box-shadow 0.3s ease;
}

.site-header.scrolled {
  border-bottom-color: var(--color-border);
  box-shadow: var(--shadow-sm);
}

.site-header-inner {
  display: flex;
  align-items: center;
  height: var(--header-height);
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 var(--content-padding);
  transition: height 0.3s ease;
}

.site-header.scrolled .site-header-inner {
  height: var(--header-height-scrolled);
}

.logo {
  font-size: 1.25rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  color: var(--color-text);
  flex-shrink: 0;
  margin-right: 1.5rem;
}

.logo:hover {
  color: var(--color-primary);
}

.search-wrapper {
  flex: 1;
  max-width: 320px;
  margin: 0 auto;
  position: relative;
}

.search-input {
  width: 100%;
  height: 40px;
  padding: 0 1rem 0 2.5rem;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-full);
  background: var(--color-card);
  color: var(--color-text);
  font-size: var(--size-caption);
  outline: none;
  transition: border-color 0.2s, box-shadow 0.2s;
}

.search-input::placeholder {
  color: var(--color-text-tertiary);
}

.search-input:focus {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.1);
}

.search-wrapper::before {
  content: '';
  position: absolute;
  left: 0.75rem;
  top: 50%;
  transform: translateY(-50%);
  width: 16px;
  height: 16px;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='16' height='16' fill='none' viewBox='0 0 24 24' stroke='%239CA3AF' stroke-width='2'%3E%3Cpath stroke-linecap='round' stroke-linejoin='round' d='M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: center;
  pointer-events: none;
}

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.5rem;
  padding: 0.625rem 1.25rem;
  font-size: var(--size-caption);
  font-weight: 600;
  border-radius: var(--radius-md);
  transition: all 0.2s ease;
  white-space: nowrap;
  flex-shrink: 0;
}

.btn:focus-visible {
  outline: 2px solid var(--color-primary);
  outline-offset: 2px;
}

.btn-primary {
  background: var(--color-primary);
  color: #fff;
  border: 1px solid var(--color-primary);
}

.btn-primary:hover {
  background: var(--color-primary-hover);
  border-color: var(--color-primary-hover);
}

.btn-primary:active {
  background: var(--color-primary-active);
  border-color: var(--color-primary-active);
}

.btn-secondary {
  background: var(--color-card);
  color: var(--color-text);
  border: 1px solid var(--color-border);
}

.btn-secondary:hover {
  border-color: var(--color-text-tertiary);
  background: #F9FAFB;
}

.btn-secondary:active {
  background: #F3F4F6;
}

.btn-ghost {
  background: transparent;
  color: var(--color-text-secondary);
  padding: 0.5rem 0.75rem;
  border-radius: var(--radius-sm);
}

.btn-ghost:hover {
  background: #F3F4F6;
  color: var(--color-text);
}

.btn-sm {
  padding: 0.375rem 0.75rem;
  font-size: var(--size-small);
}

.hero {
  padding: 5rem 0 3rem;
  text-align: center;
}

@media (min-width: 768px) {
  .hero {
    padding: 7rem 0 4rem;
  }
}

.hero-content {
  max-width: 720px;
  margin: 0 auto;
}

.hero-title {
  font-size: var(--size-hero);
  font-weight: 800;
  letter-spacing: -0.03em;
  line-height: 1.1;
  margin-bottom: 1.25rem;
  color: var(--color-text);
}

.hero-highlight {
  color: var(--color-primary);
}

.hero-subtitle {
  font-size: 1.125rem;
  line-height: 1.7;
  color: var(--color-text-secondary);
  max-width: 560px;
  margin: 0 auto 2rem;
}

@media (min-width: 768px) {
  .hero-subtitle {
    font-size: 1.25rem;
  }
}

.hero-actions {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 0.75rem;
  flex-wrap: wrap;
}

.hero-actions .btn {
  font-size: var(--size-body);
  padding: 0.75rem 1.75rem;
}

.section {
  padding: 2rem 0 4rem;
}

.section-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 1.5rem;
}

.section-title {
  font-size: var(--size-section);
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--color-text);
}

.video-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 1.5rem;
}

@media (min-width: 480px) {
  .video-grid {
    grid-template-columns: repeat(2, 1fr);
  }
}

@media (min-width: 1024px) {
  .video-grid {
    grid-template-columns: repeat(3, 1fr);
  }
}

@media (min-width: 1280px) {
  .video-grid {
    grid-template-columns: repeat(4, 1fr);
  }
}

.video-card {
  background: var(--color-card);
  border-radius: var(--radius-md);
  overflow: hidden;
  border: 1px solid var(--color-border);
  transition: transform 0.3s ease, box-shadow 0.3s ease;
}

.video-card:hover {
  transform: translateY(-4px) scale(1.02);
  box-shadow: var(--shadow-lg);
}

.video-card-link {
  display: block;
  text-decoration: none;
  color: inherit;
}

.video-card-link:focus-visible {
  outline: 2px solid var(--color-primary);
  outline-offset: 2px;
  border-radius: var(--radius-md);
}

.video-card-thumbnail {
  position: relative;
  aspect-ratio: 16 / 9;
  overflow: hidden;
  background: #F3F4F6;
}

.video-card-thumbnail img {
  width: 100%;
  height: 100%;
  object-fit: cover;
  transition: transform 0.4s ease;
}

.video-card:hover .video-card-thumbnail img {
  transform: scale(1.05);
}

.video-card-play {
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--color-overlay);
  opacity: 0;
  transition: opacity 0.3s ease;
}

.video-card:hover .video-card-play {
  opacity: 1;
}

.video-card-play-icon {
  width: 48px;
  height: 48px;
  border-radius: 50%;
  background: rgba(255, 255, 255, 0.9);
  display: flex;
  align-items: center;
  justify-content: center;
  transition: transform 0.3s ease, background 0.3s ease;
}

.video-card:hover .video-card-play-icon {
  transform: scale(1.1);
  background: #fff;
}

.video-card-play-icon svg {
  width: 20px;
  height: 20px;
  fill: var(--color-text);
  margin-left: 2px;
}

.video-card-body {
  padding: 1rem;
}

.video-card-title {
  font-size: var(--size-card-title);
  font-weight: 600;
  line-height: 1.3;
  margin-bottom: 0.375rem;
  color: var(--color-text);
  display: -webkit-box;
  -webkit-line-clamp: 2;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.video-card-desc {
  font-size: var(--size-caption);
  color: var(--color-text-secondary);
  line-height: 1.5;
  margin-bottom: 0.75rem;
  display: -webkit-box;
  -webkit-line-clamp: 2;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.video-card-date {
  font-size: var(--size-small);
  color: var(--color-text-tertiary);
}

.video-detail {
  padding: 2rem 0 4rem;
  animation: fadeIn 0.3s ease;
}

.video-detail-back {
  display: inline-flex;
  align-items: center;
  gap: 0.375rem;
  font-size: var(--size-caption);
  font-weight: 500;
  color: var(--color-text-secondary);
  margin-bottom: 1.5rem;
  transition: color 0.2s;
}

.video-detail-back:hover {
  color: var(--color-primary);
}

.video-detail-back svg {
  width: 16px;
  height: 16px;
}

.video-detail-title {
  font-size: clamp(1.5rem, 3vw, 2.25rem);
  font-weight: 800;
  letter-spacing: -0.02em;
  line-height: 1.2;
  margin-bottom: 1.5rem;
  color: var(--color-text);
}

.video-player-wrapper {
  position: relative;
  width: 100%;
  aspect-ratio: 16 / 9;
  border-radius: var(--radius-md);
  overflow: hidden;
  background: #000;
  margin-bottom: 1.5rem;
  box-shadow: var(--shadow-lg);
}

.video-player-wrapper iframe {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  border: 0;
}

.video-meta {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  padding-bottom: 1.5rem;
  border-bottom: 1px solid var(--color-border);
  margin-bottom: 1.5rem;
  flex-wrap: wrap;
}

.video-date {
  font-size: var(--size-caption);
  color: var(--color-text-secondary);
}

.video-description {
  font-size: var(--size-body);
  line-height: 1.7;
  color: var(--color-text-secondary);
  margin-bottom: 3rem;
  max-width: 720px;
}

.related-section {
  padding-top: 2rem;
  border-top: 1px solid var(--color-border);
}

.related-title {
  font-size: var(--size-section);
  font-weight: 700;
  letter-spacing: -0.02em;
  margin-bottom: 1.5rem;
  color: var(--color-text);
}

.page-404 {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  text-align: center;
  padding: 6rem 1.5rem;
  min-height: 60vh;
}

.page-404-code {
  font-size: 6rem;
  font-weight: 800;
  letter-spacing: -0.04em;
  color: var(--color-text-tertiary);
  line-height: 1;
  margin-bottom: 1rem;
}

.page-404-title {
  font-size: 1.5rem;
  font-weight: 700;
  margin-bottom: 0.75rem;
}

.page-404-desc {
  font-size: var(--size-body);
  color: var(--color-text-secondary);
  margin-bottom: 2rem;
  max-width: 400px;
}

.highlight {
  background: rgba(37, 99, 235, 0.12);
  color: var(--color-primary);
  border-radius: 2px;
  padding: 0 2px;
}

.toast {
  position: fixed;
  bottom: 2rem;
  left: 50%;
  transform: translateX(-50%) translateY(100px);
  padding: 0.75rem 1.5rem;
  background: var(--color-text);
  color: #fff;
  font-size: var(--size-caption);
  font-weight: 500;
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-xl);
  z-index: 200;
  opacity: 0;
  transition: transform 0.3s ease, opacity 0.3s ease;
  pointer-events: none;
}

.toast.visible {
  transform: translateX(-50%) translateY(0);
  opacity: 1;
}

.site-footer {
  border-top: 1px solid var(--color-border);
  padding: 1.5rem 0;
  margin-top: 2rem;
}

.site-footer-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 var(--content-padding);
  text-align: center;
}

.site-footer p {
  font-size: var(--size-caption);
  color: var(--color-text-tertiary);
}

.no-results {
  grid-column: 1 / -1;
  text-align: center;
  padding: 3rem 1rem;
  color: var(--color-text-secondary);
}

.no-results p {
  font-size: 1.125rem;
}

@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

:focus-visible {
  outline: 2px solid var(--color-primary);
  outline-offset: 2px;
}

.video-card-link:focus-visible,
.btn:focus-visible,
a:focus-visible {
  outline: 2px solid var(--color-primary);
  outline-offset: 2px;
}

@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(8px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

@keyframes fadeInUp {
  from {
    opacity: 0;
    transform: translateY(16px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.fade-in {
  animation: fadeIn 0.4s ease forwards;
}

.fade-in-up {
  animation: fadeInUp 0.5s ease forwards;
}

@media (max-width: 479px) {
  .site-header-inner {
    gap: 0.5rem;
  }

  .search-wrapper {
    max-width: 160px;
  }

  .hero-actions .btn {
    width: 100%;
  }

  .video-meta {
    flex-direction: column;
    align-items: flex-start;
  }
}

@media (min-width: 480px) and (max-width: 767px) {
  .search-wrapper {
    max-width: 200px;
  }
}

@media (max-width: 359px) {
  .search-wrapper {
    display: none;
  }
}

@media (min-width: 1600px) {
  :root {
    --max-width: 1440px;
  }

  .hero {
    padding: 8rem 0 5rem;
  }
}

@media (min-width: 1920px) {
  :root {
    --max-width: 1600px;
  }

  .video-grid {
    gap: 2rem;
  }
}