:root {
  --ink: #1d1d1f;
  --muted: #6e6e73;
  --line: #dedede;
  --soft: #f6f6f4;
  --accent: #0f6b62;
  --accent-soft: #e8f2ef;
  --paper: #fffefa;
  --reading-max: 720px;
  --figure-max: 1080px;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  color: var(--ink);
  background: var(--paper);
}

* { box-sizing: border-box; }
body { margin: 0; background: var(--paper); }
a { color: inherit; }
.visually-hidden { position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden; clip: rect(0, 0, 0, 0); white-space: nowrap; border: 0; }
.landing { width: min(960px, calc(100% - 48px)); margin: 0 auto; padding: 13vh 0 96px; }
.eyebrow { margin: 0 0 12px; color: var(--accent); font-size: 0.76rem; font-weight: 700; letter-spacing: 0.08em; text-transform: uppercase; }
h1, h2, h3, p { margin-top: 0; }
h1 { max-width: 14ch; margin-bottom: 24px; font-family: Georgia, "Times New Roman", serif; font-size: clamp(2.8rem, 7vw, 5.8rem); font-weight: 500; letter-spacing: -0.055em; line-height: 0.95; }
h2 { margin: 56px 0 14px; font-family: Georgia, "Times New Roman", serif; font-size: clamp(1.6rem, 3vw, 2.15rem); font-weight: 500; letter-spacing: -0.035em; line-height: 1.06; }
h3 { margin: 0; font-size: 1.05rem; }
p, li, td, th, summary { font-size: 1rem; line-height: 1.65; }
.landing-intro, .deck { max-width: 620px; color: #3d3d40; font-size: 1.22rem; line-height: 1.55; }
.chapter-list { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 12px; margin-top: 64px; }
.chapter-card { min-height: 170px; padding: 24px; border: 1px solid var(--line); border-radius: 12px; text-decoration: none; }
.chapter-card.current { background: var(--accent); color: white; }
.chapter-card.forthcoming { color: var(--muted); }
.chapter-number, .chapter-title, .chapter-detail { display: block; }
.chapter-number { margin-bottom: 18px; font-size: 0.78rem; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; }
.chapter-title { font-family: Georgia, "Times New Roman", serif; font-size: 1.55rem; }
.chapter-detail { margin-top: 8px; font-size: 0.9rem; line-height: 1.35; }
.site-shell { display: grid; grid-template-columns: 280px minmax(0, 1fr); min-height: 100vh; transition: grid-template-columns 220ms ease; }
.sidebar { position: sticky; top: 0; align-self: start; min-width: 0; height: 100vh; padding: 30px 24px; border-right: 1px solid var(--line); overflow: hidden; transition: opacity 180ms ease, transform 220ms ease; }
.sidebar-open, .sidebar-close { display: grid; width: 34px; height: 34px; padding: 7px; border: 1px solid var(--line); border-radius: 8px; color: var(--ink); background: color-mix(in srgb, var(--paper) 92%, transparent); cursor: pointer; box-shadow: 0 2px 8px rgb(0 0 0 / 6%); }
.sidebar-open:hover, .sidebar-close:hover { background: white; }
.sidebar-open svg, .sidebar-close svg { width: 100%; height: 100%; }
.sidebar-open { position: fixed; top: 18px; left: 18px; z-index: 10; display: none; }
.sidebar-close { position: absolute; top: 22px; right: 18px; }
.navigation-collapsed .site-shell { grid-template-columns: 0 minmax(0, 1fr); }
.navigation-collapsed .sidebar { pointer-events: none; opacity: 0; transform: translateX(-100%); }
.navigation-collapsed .sidebar-open { display: grid; }
.wordmark { display: inline-block; margin-bottom: 52px; color: var(--ink); font-family: Georgia, "Times New Roman", serif; font-size: 1.25rem; font-weight: 700; line-height: 0.94; text-decoration: none; }
.nav-section { margin: 0; padding: 14px 10px 7px; color: var(--ink); font-size: 0.77rem; font-weight: 700; letter-spacing: 0.04em; text-transform: uppercase; }
.nav-section span { display: block; margin-top: 4px; color: var(--muted); font-size: 0.72rem; font-weight: 500; letter-spacing: 0; text-transform: none; }
.nav-section.muted { color: #a0a0a2; }
.nav-link { display: block; padding: 9px 10px; border-radius: 7px; color: #424247; font-size: 0.85rem; line-height: 1.3; text-decoration: none; }
.nav-link.active { background: var(--accent-soft); color: #12554e; font-weight: 700; }
.nav-link.disabled { color: #a0a0a2; }
.reading-column { width: min(var(--reading-max), calc(100% - 72px)); margin: 0 auto; padding: 92px 0 120px; }
.page-header { padding-bottom: 36px; border-bottom: 1px solid var(--line); }
.page-header h1 { max-width: none; font-size: clamp(2.6rem, 5vw, 4.6rem); }
.section-intro { margin-bottom: 22px; color: #3d3d40; font-size: 1.1rem; line-height: 1.55; }
.model-card, .checks { margin-top: 46px; padding: 28px 30px; background: var(--soft); border-radius: 12px; }
.model-card h2, .checks h2 { margin-top: 0; }
.equation-block { overflow-x: auto; margin: 20px 0; padding: 18px; background: white; border: 1px solid var(--line); border-radius: 8px; text-align: center; }
table { width: 100%; margin: 22px 0; border-collapse: collapse; }
th, td { padding: 11px 8px; border-bottom: 1px solid var(--line); text-align: left; vertical-align: top; }
th { color: var(--muted); font-size: 0.8rem; font-weight: 700; }
ol, ul { padding-left: 1.35rem; }
.textbook-figure { width: min(var(--figure-max), calc(100vw - 352px)); margin: 34px 0 28px 50%; transform: translateX(-50%); }
.textbook-figure img { display: block; width: 100%; border: 1px solid var(--line); border-radius: 10px; background: var(--paper); }
.textbook-figure figcaption { max-width: var(--reading-max); margin-top: 10px; color: var(--muted); font-size: 0.9rem; line-height: 1.5; }
.interactive-explorer { width: min(var(--figure-max), calc(100vw - 352px)); margin: 36px 0 26px 50%; transform: translateX(-50%); }
.interactive-explorer h3 { margin-bottom: 8px; font-family: Georgia, "Times New Roman", serif; font-size: 1.45rem; font-weight: 500; letter-spacing: -0.025em; }
.interactive-explorer > p { max-width: var(--reading-max); }
.interactive-explorer iframe { display: block; width: 100%; height: 760px; border: 1px solid var(--line); border-radius: 12px; background: var(--paper); }
.interactive-explorer iframe.ray-transfer-explorer-frame { height: 940px; }
.interactive-explorer iframe.aperture-stop-explorer-frame { height: 980px; }
.interactive-explorer iframe.pupil-viewer-explorer-frame { height: 1140px; }
.example { margin-top: 26px; padding: 24px; border: 1px solid var(--line); border-radius: 12px; }
.example-heading { display: flex; align-items: baseline; gap: 12px; margin-bottom: 18px; }
.example-label { display: inline-flex; align-items: center; gap: 8px; color: var(--accent); font-size: 0.75rem; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; white-space: nowrap; }
.level-dots { display: inline-flex; gap: 4px; }
.level-dot { width: 8px; height: 8px; border: 1.5px solid currentcolor; border-radius: 50%; }
.level-dot.is-active { background: currentcolor; }
.prediction-prompt { margin-top: 26px; padding: 24px; border-left: 4px solid var(--accent); background: var(--accent-soft); border-radius: 0 12px 12px 0; }
.prediction-prompt .eyebrow { margin-bottom: 8px; }
.prediction-prompt h3 { margin-bottom: 10px; }
details { margin-top: 12px; border-top: 1px solid var(--line); }
summary { padding: 13px 0; color: var(--accent); font-weight: 700; cursor: pointer; }
details > :last-child { padding-bottom: 4px; }
.checks ul { margin-bottom: 0; }
.page-resource { margin: 20px 0 0; font-size: 0.95rem; }
.page-resource a { color: var(--accent); font-weight: 700; }
.lecture-deck { min-height: 100vh; padding: 18px; background: #153934; color: #fffefa; }
.deck-toolbar { display: flex; align-items: center; justify-content: space-between; gap: 16px; width: min(1320px, 100%); margin: 0 auto 16px; }
.deck-home { color: inherit; font-size: 0.88rem; font-weight: 700; text-decoration: none; }
.deck-controls { display: flex; align-items: center; gap: 10px; }
.deck-controls button { min-width: 88px; padding: 9px 13px; border: 1px solid rgb(255 255 255 / 42%); border-radius: 8px; color: inherit; background: transparent; font: inherit; cursor: pointer; }
.deck-controls button:disabled { opacity: 0.35; cursor: default; }
.deck-counter { min-width: 44px; color: #c9e2db; font-variant-numeric: tabular-nums; text-align: center; }
.slide { display: grid; grid-template-columns: minmax(0, 1fr); align-content: center; width: min(1320px, 100%); min-height: calc(100vh - 96px); margin: 0 auto; padding: clamp(34px, 6vw, 92px); border: 1px solid rgb(255 255 255 / 18%); border-radius: 18px; background: #1d4a44; outline: none; }
.slide[hidden] { display: none; }
.slide h1, .slide h2 { max-width: 20ch; color: #fffefa; }
.slide h1 { font-size: clamp(3.2rem, 8vw, 7.2rem); }
.slide h2 { margin-top: 0; font-size: clamp(2.25rem, 5vw, 4.6rem); }
.slide p, .slide li { max-width: 880px; color: #edf5f1; font-size: clamp(1.05rem, 1.6vw, 1.35rem); line-height: 1.55; }
.slide .eyebrow { color: #9bd7c8; }
.slide .equation-block { max-width: 940px; color: var(--ink); }
.slide-columns { display: grid; grid-template-columns: minmax(0, 1fr) minmax(320px, 0.95fr); gap: clamp(28px, 6vw, 92px); align-items: center; }
.slide-figure { width: 100%; margin: 0; }
.slide-figure img { display: block; width: 100%; border-radius: 12px; background: var(--paper); }
.slide-figure figcaption { margin-top: 10px; color: #c9e2db; font-size: 0.9rem; line-height: 1.45; }
.slide-prompt { margin-top: 28px; padding: 20px 22px; border-left: 4px solid #9bd7c8; background: rgb(255 255 255 / 8%); border-radius: 0 12px 12px 0; }
.slide-prompt p { margin-bottom: 0; }
.slide details { max-width: 900px; margin-top: 24px; border-top-color: rgb(255 255 255 / 28%); }
.slide summary { color: #9bd7c8; }
.slide details p, .slide details li { font-size: 1.05rem; }
.slide .model-card { max-width: 920px; margin-top: 26px; background: #fffefa; color: var(--ink); }
.slide .model-card p { color: #303336; font-size: 1.05rem; }
@media (max-width: 780px) { .lecture-deck { padding: 10px; } .deck-toolbar { margin-bottom: 10px; } .deck-controls button { min-width: 0; } .slide { min-height: calc(100vh - 78px); padding: 32px 24px; border-radius: 12px; } .slide-columns { grid-template-columns: 1fr; gap: 26px; } }
.navigation-collapsed .textbook-figure, .navigation-collapsed .interactive-explorer { width: min(var(--figure-max), calc(100vw - 72px)); }
@media (max-width: 780px) { .site-shell, .navigation-collapsed .site-shell { display: block; } .sidebar, .navigation-collapsed .sidebar { position: static; height: auto; padding: 20px; border-right: 0; border-bottom: 1px solid var(--line); pointer-events: auto; opacity: 1; transform: none; } .sidebar-open, .sidebar-close { display: none; } .wordmark { margin-bottom: 22px; } .sidebar nav { display: none; } .reading-column { width: min(100% - 40px, 760px); padding-top: 58px; } .textbook-figure, .interactive-explorer { width: min(var(--figure-max), 92vw); margin-left: 0; transform: none; } .chapter-list { grid-template-columns: 1fr; } }
@media (max-width: 620px) { .interactive-explorer iframe.aperture-stop-explorer-frame { height: 1280px; } .interactive-explorer iframe.pupil-viewer-explorer-frame { height: 1320px; } }