@media (max-width: 75rem) {
  .grid--5 { --grid-columns: 3; }
  .grid--4 { --grid-columns: 2; }

  .article-layout {
    grid-template-columns: 14rem minmax(0, 1fr);
  }

  .article-layout > .utility-rail {
    position: static;
    grid-column: 2;
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: var(--space-4);
  }

  .utility-rail .sidebar-panel + .sidebar-panel { margin-top: 0; }
}

@media (max-width: 64rem) {
  :root {
    --section-space: clamp(3.25rem, 7vw, 4.5rem);
  }

  .js .navigation-toggle { display: inline-flex; }

  .site-header__tools { gap: var(--space-3); }

  .js .site-navigation {
    position: absolute;
    top: calc(100% + var(--border-thin));
    right: var(--page-gutter);
    left: var(--page-gutter);
    display: none;
    padding: var(--space-3);
    border: var(--border-thin) solid rgb(255 255 255 / 14%);
    border-radius: var(--radius-medium);
    background: var(--color-navy-900);
    box-shadow: var(--shadow-raised);
  }

  .js .site-header[data-navigation-open="true"] .site-navigation { display: block; }

  .site-navigation__list { align-items: stretch; flex-direction: column; }
  .site-navigation a { padding: var(--space-3) var(--space-4); }
  .mega-menu { position: static; grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .mega-menu__group + .mega-menu__group { border-left: 0; padding-left: 0; }

  .page-hero__layout,
  .split-layout,
  .article-layout {
    grid-template-columns: 1fr;
  }

  .page-hero__visual { min-height: 10rem; }

  .feature-rail { grid-template-columns: repeat(2, minmax(0, 1fr)); gap: var(--space-4); }
  .feature-rail__item + .feature-rail__item { border-left: 0; }

  .course-sidebar,
  .utility-rail { position: static; }

  .article-layout > .utility-rail {
    grid-column: auto;
  }

  .architecture-grid,
  .grid--3 { --grid-columns: 2; }
}

@media (max-width: 48rem) {
  :root { --page-gutter: 1.25rem; }

  h1 { font-size: clamp(2.15rem, 10vw, 3rem); }
  .site-header__actions { display: none; }
  .page-hero { padding-block: var(--space-8); }
  .page-hero__summary { font-size: var(--font-size-base); }

  .grid,
  .grid--2,
  .grid--3,
  .grid--4,
  .grid--5,
  .architecture-grid,
  .feature-rail,
  .stats-strip,
  .form-row,
  .article-layout > .utility-rail {
    grid-template-columns: 1fr;
    --grid-columns: 1;
  }

  .feature-rail__item { padding: var(--space-2); }
  .stats-strip__item + .stats-strip__item { border-top: var(--border-thin) solid rgb(255 255 255 / 12%); border-left: 0; }

  .search-form { align-items: stretch; flex-direction: column; padding: var(--space-2); }
  .search-form input { min-height: 2.75rem; }
  .search-form select { min-height: 2.75rem; border-top: var(--border-thin) solid var(--color-border); border-left: 0; }
  .search-form .button { width: 100%; }

  .cta-panel { align-items: flex-start; flex-direction: column; }

  .site-footer__grid { grid-template-columns: repeat(2, minmax(0, 1fr)); gap: var(--space-6); }
}

@media (max-width: 36rem) {
  :root { --page-gutter: 0.75rem; }

  .architecture-shell,
  .error-shell { padding-block: var(--space-6); }

  .status-card { padding: var(--space-6) var(--space-5); border-radius: var(--radius-medium); }
  .card, .panel, .architecture-card { padding: var(--space-4); }
  .mega-menu { grid-template-columns: 1fr; }

  .site-footer__grid { grid-template-columns: 1fr; }
  .site-footer__inner { padding-block: var(--space-4); align-items: flex-start; flex-direction: column; justify-content: center; gap: var(--space-1); }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }

  *,
  *::before,
  *::after {
    scroll-behavior: auto !important;
    transition-duration: 0.01ms !important;
  }
}

@media (forced-colors: active) {
  .button,
  .badge,
  .number-tile,
  .icon-tile { border: var(--border-thin) solid currentColor; }
}

/* Phase 4 homepage responsive states */
@media (max-width: 87.5rem) {
  .home-header .site-navigation a { padding-inline: 0.5rem; font-size: 0.84rem; }
  .home-header .site-header__actions .button { padding-inline: 1rem; }
  .home-hero__layout { grid-template-columns: minmax(0, 1.05fr) minmax(30rem, 0.95fr); gap: 1.5rem; }
  .home-benefits { padding-inline: 1.3rem; }
  .home-benefit { padding-inline: 0.45rem; gap: 0.75rem; }
  .benefit-icon { width: 3.25rem; height: 3.25rem; }
  .feature-panel { padding-inline: 1.35rem; }
}

@media (max-width: 75rem) {
  .home-hero h1 { font-size: clamp(2.7rem, 4.45vw, 3.35rem); }
  .home-benefits { grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 1.4rem; }
  .tutorial-grid { grid-template-columns: repeat(3, minmax(0, 1fr)); }
  .tutorial-card { min-height: 16rem; }
  .home-stats { grid-template-columns: repeat(3, minmax(0, 1fr)); }
  .home-stat:nth-child(4) { border-left: 0; }
  .home-stat:nth-child(n+4) { border-top: 1px solid rgb(255 255 255 / 13%); }
  .home-feature-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .tools-panel { grid-column: 1 / -1; min-height: auto; }
  .tool-list { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .testimonial-card { padding-inline: 1.1rem; }
  .testimonial-card header { grid-template-columns: 2.8rem minmax(0, 1fr); }
  .testimonial-card .stars { grid-column: 2; }
}

@media (max-width: 64rem) {
  .home-header .site-header__inner { min-height: 5rem; }
  .home-header .brand__logo { height: 3.25rem; }
  .home-header .site-header__tools { flex: 0 0 auto; }
  .home-header .site-navigation { z-index: 2; }
  .home-header .site-navigation__list { gap: 0; }
  .home-header .site-navigation a { padding: 0.8rem 1rem; font-size: 0.9rem; }
  .home-hero,
  .home-hero__layout { min-height: auto; }
  .home-hero__layout { grid-template-columns: 1fr; padding-top: 4rem; padding-bottom: 6.5rem; }
  .home-hero__content { padding-top: 0; }
  .home-hero__summary br { display: none; }
  .hero-art { width: min(100%, 46rem); height: 28rem; margin: 0 auto; }
  .home-benefits { margin-top: -4.3rem; }
  .home-feature-grid { grid-template-columns: 1fr; }
  .tools-panel { grid-column: auto; }
  .feature-panel { min-height: auto; }
  .newsletter-panel { grid-template-columns: 4rem 1fr; }
  .newsletter-form { grid-column: 1 / -1; }
}

@media (max-width: 48rem) {
  .home-header .site-header__inner { gap: 0.75rem; }
  .home-header .brand__logo { height: 2.85rem; }
  .home-header .site-header__tools { display: block; }
  .home-hero__layout { padding-top: 3rem; padding-bottom: 5.5rem; }
  .home-hero h1 { font-size: clamp(2.2rem, 9vw, 3.05rem); }
  .home-hero__summary { font-size: 1rem; }
  .home-search { min-height: auto; }
  .home-search__icon { display: none; }
  .home-search select { border-top: 1px solid var(--color-border); }
  .hero-art { height: 23rem; transform: scale(0.94); transform-origin: center top; }
  .floating-tile--cloud { left: 0; }
  .floating-tile--code { right: 5%; }
  .home-benefits { padding: 1.5rem; grid-template-columns: 1fr; }
  .home-benefit { padding: 0.5rem; }
  .home-tutorials { padding-top: 2.6rem; }
  .tutorial-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .tutorial-card { min-height: 17rem; }
  .home-stats { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .home-stat:nth-child(4) { border-left: 1px solid rgb(255 255 255 / 13%); }
  .home-stat:nth-child(3),
  .home-stat:nth-child(5) { border-left: 0; }
  .home-stat:nth-child(n+3) { border-top: 1px solid rgb(255 255 255 / 13%); }
  .home-feature-grid { gap: 1.25rem; }
  .seo-panel__content { min-height: 17rem; }
  .tool-list { grid-template-columns: 1fr; }
  .testimonial-grid { grid-template-columns: 1fr; }
  .testimonial-card header { grid-template-columns: 2.8rem minmax(0, 1fr) auto; }
  .testimonial-card .stars { grid-column: auto; }
  .newsletter-panel { grid-template-columns: 3.5rem 1fr; padding-inline: 1.2rem; }
  .newsletter-form { min-width: 0; }
  .newsletter-form .button { min-width: 7.5rem; }
  .home-footer .site-footer__grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }
}

@media (max-width: 36rem) {
  .home-header .brand__logo { height: 2.5rem; }
  .home-header .navigation-toggle { min-height: 2.5rem; padding-inline: 0.7rem; }
  .home-header .navigation-toggle__label { font-size: 0.8rem; }
  .hero-kicker { font-size: 0.68rem; }
  .home-hero h1 { font-size: clamp(1.95rem, 9.5vw, 2.65rem); }
  .hero-art { height: 19rem; margin-top: 0.5rem; }
  .floating-panel { right: 7%; width: 6.5rem; height: 5rem; }
  .floating-tile--cloud { width: 4rem; height: 3.3rem; }
  .floating-tile--chart { left: 7%; width: 4.5rem; }
  .floating-tile--code { width: 4.5rem; height: 4.1rem; }
  .database { left: 1%; width: 4.8rem; transform: scale(0.8); transform-origin: left bottom; }
  .rocket { right: 0; transform: rotate(27deg) scale(0.75); transform-origin: bottom; }
  .books { right: 0; transform: rotate(-5deg) scale(0.74); transform-origin: right bottom; }
  .plant { right: 23%; transform: scale(0.72); transform-origin: bottom; }
  .tutorial-grid { grid-template-columns: 1fr; }
  .tutorial-card { min-height: 15.5rem; }
  .home-stats { grid-template-columns: 1fr; }
  .home-stat { min-height: 4.5rem; }
  .home-stat + .home-stat { border-top: 1px solid rgb(255 255 255 / 13%); border-left: 0; }
  .seo-panel__content { grid-template-columns: 1fr; padding-block: 1.5rem; }
  .score-gauge { width: 8.5rem; }
  .score-gauge div { top: 2rem; }
  .seo-actions { align-items: stretch; flex-direction: column; gap: 0.7rem; }
  .seo-actions .button { width: 100%; }
  .question { grid-template-columns: 2.6rem minmax(0, 1fr); }
  .question em { grid-column: 2; justify-self: start; }
  .testimonial-card header { grid-template-columns: 2.8rem minmax(0, 1fr); }
  .testimonial-card .stars { grid-column: 2; }
  .newsletter-panel { grid-template-columns: 3.5rem 1fr; }
  .newsletter-form { flex-direction: column; border: 0; background: transparent; gap: 0.6rem; overflow: visible; }
  .newsletter-form input { min-height: 3.2rem; border: 1px solid #f1c8ad; border-radius: 0.55rem; background: #fff; }
  .newsletter-form .button { width: 100%; }
  .home-footer .site-footer__grid { grid-template-columns: 1fr; }
}

/* Phase 5 mega navigation responsive states */
@media (max-width: 87.5rem) and (min-width: 64.01rem) {
  .site-navigation__trigger,
  .site-header--mega .site-navigation__link {
    padding-inline: 0.5rem;
    font-size: 0.84rem;
  }

  .site-header--mega .site-header__actions .button {
    padding-inline: 1rem;
  }

  .site-header--mega .mega-menu {
    width: calc(100vw - 4rem);
  }

  .mega-menu__grid {
    gap: 2rem;
  }
}

@media (max-width: 64rem) {
  .site-header--mega .site-header__inner {
    min-height: 5rem;
    gap: 0.75rem;
  }

  .site-header--mega .brand__logo {
    height: 3.25rem;
  }

  .site-header__mobile-controls {
    display: flex;
  }

  .theme-toggle--desktop {
    display: none;
  }

  .theme-toggle--mobile {
    display: inline-grid;
  }

  .site-header--mega .site-header__tools {
    position: absolute;
    top: calc(100% + 0.0625rem);
    right: var(--page-gutter);
    left: var(--page-gutter);
    z-index: var(--z-menu);
    max-height: calc(100vh - 6rem);
    padding: 0.75rem;
    display: none;
    flex-direction: column;
    align-items: stretch;
    gap: 0.75rem;
    overflow-y: auto;
    border: 1px solid rgb(255 255 255 / 14%);
    border-radius: 0 0 0.75rem 0.75rem;
    background: #031735;
    box-shadow: var(--shadow-raised);
  }

  .site-header--mega[data-navigation-open="true"] .site-header__tools {
    display: flex;
  }

  .js .site-header--mega .site-navigation {
    position: static;
    width: 100%;
    padding: 0;
    display: block;
    border: 0;
    border-radius: 0;
    background: transparent;
    box-shadow: none;
  }

  .site-header--mega .site-navigation__list {
    gap: 0.15rem;
  }

  .site-navigation__item {
    width: 100%;
  }

  .site-navigation__trigger,
  .site-header--mega .site-navigation__link {
    width: 100%;
    min-height: 2.85rem;
    padding: 0.75rem 0.9rem;
    justify-content: space-between;
    font-size: 0.9rem;
  }

  .site-header--mega .site-header__actions {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 0.75rem;
  }

  .site-header--mega .site-header__actions li,
  .site-header--mega .site-header__actions .button {
    width: 100%;
  }

  .site-header--mega .mega-menu {
    width: calc(100vw - (2 * var(--page-gutter)));
    max-height: calc(100vh - 5rem);
    padding: 1rem 1rem 0;
    border-radius: 0 0 0.75rem 0.75rem;
  }

  .mega-menu__mobile-close {
    position: sticky;
    top: -1rem;
    z-index: 2;
    width: 100%;
    min-height: 3rem;
    margin: -0.15rem 0 1rem;
    padding: 0.65rem 0.75rem;
    display: flex;
    align-items: center;
    gap: 0.55rem;
    border: 1px solid var(--color-border);
    border-radius: 0.5rem;
    background: var(--color-surface);
    color: var(--color-text);
    font-weight: var(--font-weight-semibold);
    cursor: pointer;
  }

  .mega-menu__grid {
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 1.75rem 2.25rem;
  }

  .mega-menu__footer {
    padding-bottom: 1rem;
  }
}

@media (max-width: 40rem) {
  .site-header--mega .brand__logo {
    height: 2.7rem;
  }

  .site-header--mega .navigation-toggle {
    min-height: 2.5rem;
    padding-inline: 0.7rem;
  }

  .site-header--mega .navigation-toggle__label {
    font-size: 0.8rem;
  }

  .mega-menu__grid {
    grid-template-columns: 1fr;
    gap: 1.5rem;
  }

  .mega-menu__footer {
    align-items: stretch;
    flex-direction: column;
    gap: 1rem;
  }

  .mega-menu__cta {
    width: 100%;
  }
}

@media (max-width: 23.5rem) {
  .site-header--mega .site-header__inner {
    gap: 0.4rem;
  }

  .site-header--mega .brand__logo {
    height: 2.35rem;
  }

  .site-header__mobile-controls {
    gap: 0.35rem;
  }

  .site-header--mega .theme-toggle {
    width: 1.8rem;
    height: 1.8rem;
  }

  .site-header--mega .navigation-toggle__label {
    position: absolute;
    width: 1px;
    height: 1px;
    overflow: hidden;
    clip: rect(0 0 0 0);
    clip-path: inset(50%);
    white-space: nowrap;
  }
}

/* Phase 6 system-page responsive states */
@media (max-width: 64rem) {
  .system-main {
    min-height: auto;
    padding-block: 4rem;
  }

  .system-card {
    width: min(100%, 46rem);
    grid-template-columns: 1fr;
  }

  .system-card__visual {
    min-height: 22rem;
  }
}

@media (max-width: 48rem) {
  .system-main {
    padding-block: 2.5rem;
  }

  .system-card {
    border-radius: 0.9rem;
  }

  .system-card__copy {
    padding: 2.25rem 1.5rem;
  }

  .system-card h1 {
    font-size: clamp(2.4rem, 12vw, 3.5rem);
  }

  .system-card__summary {
    font-size: 1rem;
  }

  .system-actions {
    width: 100%;
    align-items: stretch;
    flex-direction: column;
  }

  .system-actions .button {
    width: 100%;
  }

  .system-card__visual {
    min-height: 19rem;
  }

  .system-orbit--outer { width: 19rem; }
  .system-orbit--inner { width: 13rem; }
  .system-visual-mark { width: 6.25rem; height: 6.25rem; font-size: 1.7rem; }
}