@import url('https://fonts.googleapis.com/css2?family=Raleway:ital,wght@0,300;0,500;0,700;0,800;1,400&family=IBM+Plex+Mono:wght@400;600&family=Inter:wght@300;400;500;600&display=swap');

:root {
  --bg:      #f7f7f8;
  --bg2:     #eeeef0;
  --card:    #ffffff;
  --border:  #d8d8dc;
  --red:     #c0392b;
  --red2:    #e74c3c;
  --slate:   #2c3e50;
  --slate2:  #3d5166;
  --teal:    #16a085;
  --amber:   #d35400;
  --text:    #1a1a24;
  --text2:   #52526a;
  --text3:   #8888a0;
  --muted:   #c8c8d8;
}

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

body {
  font-family: 'Inter', sans-serif;
  background: var(--bg);
  color: var(--text);
  overflow-x: hidden;
}

/* ── HEADER ──────────────────────── */
.dv-header {
  background: linear-gradient(135deg, var(--red) 0%, #9b1a1a 100%);
  position: relative; overflow: hidden;
}

/* Marquee band */
.dv-header-marquee {
  background: rgba(0,0,0,.2);
  padding: 6px 0; overflow: hidden; white-space: nowrap;
  display: flex; gap: 24px; animation: marqueeScroll 22s linear infinite;
  font-family: 'IBM Plex Mono', monospace; font-size: 0.82rem; color: rgba(255,255,255,.6);
}
@keyframes marqueeScroll { from { transform: translateX(0); } to { transform: translateX(-50%); } }
.dv-header-marquee span { padding: 0 6px; flex-shrink: 0; }

.dv-header-inner {
  max-width: 960px; margin: 0 auto; padding: 40px 32px 0; position: relative;
}

.dv-dept { font-size: 0.68rem; letter-spacing: 0.22em; text-transform: uppercase; color: rgba(255,255,255,.55); margin-bottom: 16px; font-weight: 600; }

.dv-title {
  font-family: 'Raleway', sans-serif;
  font-size: clamp(3rem, 10vw, 6.5rem);
  font-weight: 800;
  color: #fff;
  letter-spacing: -0.04em;
  line-height: 0.85;
}

.dv-tagline { margin-top: 12px; color: rgba(255,255,255,.7); font-size: 0.92rem; }

.dv-hero-canvas {
  display: block; width: 100%; height: 140px;
  margin-top: 20px; opacity: 0.6;
}

/* ── NAV ─────────────────────────── */
.dv-nav {
  position: sticky; top: 0; z-index: 100;
  background: var(--slate); backdrop-filter: none;
  display: flex; flex-wrap: wrap; justify-content: center;
}
.dv-nav-a {
  padding: 11px 14px; font-size: 0.77rem; font-weight: 500;
  text-decoration: none; color: rgba(255,255,255,.55);
  border-bottom: 2px solid transparent; transition: all .2s; white-space: nowrap;
}
.dv-nav-a:hover { color: #fff; border-color: var(--red2); }
.dv-nav-a.active { color: #fff; border-color: var(--red2); }

/* ── MAIN ────────────────────────── */
.dv-main { max-width: 1000px; margin: 0 auto; padding: 48px 24px 80px; display: flex; flex-direction: column; gap: 52px; }

/* ── SECTION ─────────────────────── */
.dv-section { opacity: 0; transform: translateY(24px); transition: opacity .5s, transform .5s; }
.dv-section.visible { opacity: 1; transform: translateY(0); }

.dv-sec-head {
  display: flex; align-items: center; gap: 14px;
  padding: 13px 22px; border-radius: 8px 8px 0 0;
}
.dv-sec-head.red   { background: var(--red); }
.dv-sec-head.slate { background: var(--slate); }

.dv-n { font-family: 'IBM Plex Mono', monospace; font-size: 0.72rem; color: rgba(255,255,255,.45); }
.dv-sec-head h2 { font-family: 'Raleway', sans-serif; font-size: 1.2rem; font-weight: 700; color: #fff; }

.dv-sec-body {
  background: var(--card); border: 1px solid var(--border);
  border-top: none; border-radius: 0 0 8px 8px; padding: 28px;
  box-shadow: 0 3px 12px rgba(0,0,0,.05);
}
.dv-sec-body p { color: var(--text2); line-height: 1.85; font-size: 0.95rem; margin-bottom: 16px; }
.dv-sec-body p:last-child { margin-bottom: 0; }
.dv-sec-body strong { color: var(--text); }

/* ── DEF BOX ─────────────────────── */
.dv-def-box {
  background: rgba(192,57,43,.06); border: 1px solid rgba(192,57,43,.2);
  border-radius: 8px; padding: 18px 24px; margin: 18px 0; text-align: center;
}
.dv-def-label { font-size: 0.7rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.1em; color: var(--red); margin-bottom: 10px; }
.dv-def-eq { font-family: 'IBM Plex Mono', monospace; font-size: 1.05rem; color: var(--slate); }
.dv-frac { display: inline-flex; flex-direction: column; align-items: center; vertical-align: middle; gap: 2px; margin: 0 4px; }
.dv-num { border-bottom: 1px solid currentColor; padding: 0 4px; }
.dv-den { padding: 0 4px; }

/* ── INTERACTIVE CANVAS ──────────── */
.dv-interactive-canvas-wrap { margin: 18px 0; }
#dvTangentCanvas {
  display: block; width: 100%; max-width: 600px;
  border: 1px solid var(--border); border-radius: 8px; background: #fff;
  margin: 0 auto;
}
.dv-canvas-controls {
  display: flex; align-items: center; gap: 14px; margin-top: 10px;
  justify-content: center; flex-wrap: wrap;
}
.dv-canvas-controls label { font-size: 0.84rem; color: var(--text2); font-family: 'IBM Plex Mono', monospace; }
#aSlider { accent-color: var(--red); width: 160px; }
.dv-canvas-hint { font-size: 0.75rem; color: var(--text3); font-style: italic; }

/* ── DEF INTERPRET ───────────────── */
.dv-def-interpret { display: grid; grid-template-columns: repeat(3,1fr); gap: 12px; margin-top: 18px; }
.dv-di-card { background: var(--bg2); border: 1px solid var(--border); border-radius: 8px; padding: 16px; }
.dv-di-card h4 { font-size: 0.85rem; font-weight: 700; color: var(--slate); margin-bottom: 8px; }
.dv-di-card p  { font-size: 0.82rem; color: var(--text2); margin: 0; line-height: 1.7; }

/* ── NOTATION GRID ───────────────── */
.dv-notation-grid { display: grid; grid-template-columns: repeat(3,1fr); gap: 14px; }
.dv-not-card { background: var(--bg2); border: 1px solid var(--border); border-radius: 8px; padding: 18px; }
.dv-not-sym { font-family: 'IBM Plex Mono', monospace; font-size: 1.6rem; font-weight: 600; color: var(--red); margin-bottom: 4px; }
.dv-not-who { font-size: 0.72rem; font-weight: 600; text-transform: uppercase; letter-spacing: 0.08em; color: var(--text3); margin-bottom: 8px; }
.dv-not-card p  { font-size: 0.82rem; color: var(--text2); margin: 0; line-height: 1.7; }

/* ── FORMULA TABLE ───────────────── */
.dv-fml-cols { display: grid; grid-template-columns: 1fr 1fr; gap: 24px; }
.dv-fml-group-title { font-size: 0.72rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.1em; color: var(--slate); margin-bottom: 8px; }
.dv-fml-table { border: 1px solid var(--border); border-radius: 6px; overflow: hidden; }
.dv-fml-tr {
  display: grid; grid-template-columns: 1.2fr 1.5fr 1fr;
  padding: 7px 12px; border-bottom: 1px solid var(--border);
  align-items: center; transition: background .15s;
}
.dv-fml-tr:last-child { border-bottom: none; }
.dv-fml-tr:hover { background: rgba(192,57,43,.04); }
.dv-fml-tr code { font-family: 'IBM Plex Mono', monospace; font-size: 0.85rem; }
.dv-fml-tr code:first-child { color: var(--red); }
.dv-fml-tr code:nth-child(2) { color: var(--slate); font-weight: 600; }
.dv-fml-tr span { font-size: 0.75rem; color: var(--text3); }

/* ── RULES GRID ──────────────────── */
.dv-rules-grid { display: grid; grid-template-columns: repeat(2,1fr); gap: 14px; }
.dv-rule-card { background: var(--bg2); border: 1px solid var(--border); border-radius: 8px; padding: 18px; }
.dv-rule-name { font-size: 0.8rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.07em; color: var(--slate); margin-bottom: 8px; }
.dv-rule-eq { font-family: 'IBM Plex Mono', monospace; font-size: 0.95rem; color: var(--red); background: rgba(192,57,43,.07); border-radius: 5px; padding: 8px 12px; margin-bottom: 10px; display: inline-block; }
.dv-rule-card p { font-size: 0.83rem; color: var(--text2); margin: 0; line-height: 1.7; }

/* ── HIGHER DERIVATIVES ──────────── */
.dv-higher-row { display: grid; grid-template-columns: repeat(3,1fr); gap: 14px; }
.dv-high-card { background: var(--bg2); border: 1px solid var(--border); border-radius: 8px; padding: 18px; }
.dv-high-card h4 { font-size: 0.87rem; font-weight: 700; color: var(--slate); margin-bottom: 12px; }
.dv-high-rules { display: flex; flex-direction: column; gap: 8px; }
.dv-high-rules div { display: flex; gap: 10px; align-items: baseline; font-size: 0.83rem; }
.dv-high-rules code { font-family: 'IBM Plex Mono', monospace; font-size: 0.8rem; color: var(--red); flex-shrink: 0; }
.dv-high-rules span { color: var(--text2); }
.dv-high-fml { font-family: 'IBM Plex Mono', monospace; font-size: 0.9rem; color: var(--red); background: rgba(192,57,43,.06); border-radius: 5px; padding: 8px 12px; margin: 8px 0; }
.dv-high-card p  { font-size: 0.82rem; color: var(--text2); margin: 0; line-height: 1.7; }

/* ── APPLICATIONS ────────────────── */
.dv-app-grid { display: grid; grid-template-columns: repeat(3,1fr); gap: 14px; }
.dv-app-card { background: var(--bg2); border: 1px solid var(--border); border-radius: 8px; padding: 16px; }
.dv-app-card h4 { font-size: 0.85rem; font-weight: 700; color: var(--slate); margin-bottom: 8px; }
.dv-app-card p  { font-size: 0.82rem; color: var(--text2); margin: 0; line-height: 1.7; }

/* ── PARTIAL ─────────────────────── */
.dv-formula-box { border-radius: 8px; padding: 16px 20px; margin: 14px 0; text-align: center; }
.dv-formula-box.red { background: rgba(192,57,43,.06); border: 1px solid rgba(192,57,43,.2); }
.dv-fb-label { font-size: 0.7rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.1em; color: var(--text3); margin-bottom: 8px; }
.dv-fb-eq { font-family: 'IBM Plex Mono', monospace; font-size: 1rem; color: var(--red); }

.dv-partial-grid { display: grid; grid-template-columns: repeat(2,1fr); gap: 14px; margin-top: 14px; }
.dv-pa-card { background: var(--bg2); border: 1px solid var(--border); border-radius: 8px; padding: 16px; }
.dv-pa-card h4 { font-size: 0.85rem; font-weight: 700; color: var(--slate); margin-bottom: 8px; }
.dv-pa-fml { font-family: 'IBM Plex Mono', monospace; font-size: 0.92rem; color: var(--red); background: rgba(192,57,43,.06); border-radius: 4px; padding: 6px 10px; margin-bottom: 8px; display: inline-block; }
.dv-pa-card p  { font-size: 0.82rem; color: var(--text2); margin: 0; line-height: 1.7; }

/* ── OPTIM ───────────────────────── */
.dv-optim-steps { display: flex; flex-direction: column; gap: 12px; margin-bottom: 22px; }
.dv-os-step { display: flex; gap: 16px; align-items: flex-start; background: var(--bg2); border: 1px solid var(--border); border-radius: 8px; padding: 16px 20px; }
.dv-os-n { width: 32px; height: 32px; flex-shrink: 0; background: var(--red); color: #fff; border-radius: 50%; display: flex; align-items: center; justify-content: center; font-family: 'Raleway', sans-serif; font-weight: 800; font-size: 0.9rem; margin-top: 2px; }
.dv-os-step h4 { font-size: 0.9rem; font-weight: 700; color: var(--text); margin-bottom: 6px; }
.dv-os-step p  { font-size: 0.85rem; color: var(--text2); margin: 0; line-height: 1.7; }

.dv-optim-examples h4 { font-size: 0.87rem; font-weight: 700; color: var(--slate); margin-bottom: 12px; }
.dv-oe-grid { display: grid; grid-template-columns: repeat(2,1fr); gap: 12px; }
.dv-oe-card { background: rgba(192,57,43,.04); border: 1px solid rgba(192,57,43,.15); border-radius: 6px; padding: 14px 16px; font-size: 0.84rem; color: var(--text2); line-height: 1.7; }
.dv-oe-card strong { color: var(--text); }

/* ── HERO CANVAS ─────────────────── */
#dvHeroCanvas { pointer-events: none; }

/* ── FOOTER ──────────────────────── */
.dv-footer { border-top: 3px solid var(--red); background: var(--slate); padding: 20px 32px; }
.dv-footer-inner { max-width: 960px; margin: 0 auto; display: flex; align-items: center; gap: 14px; font-size: 0.82rem; color: rgba(255,255,255,.5); }
.dv-footer-sym { font-family: 'IBM Plex Mono', monospace; font-size: 1.2rem; color: var(--red2); font-weight: 600; }

/* ── RESPONSIVE ──────────────────── */
@media (max-width: 768px) {
  .dv-def-interpret, .dv-notation-grid, .dv-app-grid, .dv-higher-row { grid-template-columns: 1fr; }
  .dv-fml-cols, .dv-rules-grid, .dv-partial-grid, .dv-oe-grid { grid-template-columns: 1fr; }
  .dv-fml-tr { grid-template-columns: 1.2fr 1.5fr; }
  .dv-fml-tr span { display: none; }
}