:root {
  --bg: #f7f8fc; --card: #fff; --text: #172033; --muted: #5b6477; --line: #e2e6ef;
  --accent: #1d4ed8; --accent-2: #0ea5e9; --ok: #15803d; --bad: #b91c1c; --code: #eef2ff;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root { --bg: #0e1320; --card: #161d2e; --text: #e6e9f2; --muted: #9aa3b8; --line: #263049;
    --accent: #7aa2ff; --accent-2: #38bdf8; --ok: #4ade80; --bad: #f87171; --code: #1e2740; color-scheme: dark; }
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--text);
  font: 17px/1.65 system-ui, -apple-system, "Segoe UI", Roboto, "Noto Sans", "Noto Sans Devanagari", sans-serif; }
a { color: var(--accent); }
.wrap { max-width: 920px; margin: 0 auto; padding: 0 16px; }
.top { display: flex; justify-content: space-between; align-items: center; gap: 12px; flex-wrap: wrap;
  padding: 12px 16px; background: var(--card); border-bottom: 1px solid var(--line); position: sticky; top: 0; z-index: 5; }
.brand { font-weight: 700; text-decoration: none; color: var(--text); }
.langs a { margin-left: 10px; text-decoration: none; font-size: 15px; }
.langs a[aria-current] { font-weight: 700; text-decoration: underline; }
h1 { font-size: clamp(1.7rem, 4.5vw, 2.4rem); line-height: 1.2; margin: 18px 0 10px; }
h2 { font-size: 1.35rem; margin: 34px 0 10px; }
h3 { font-size: 1.05rem; margin: 0 0 6px; display: inline; }
.lead { font-size: 1.1rem; color: var(--muted); }
.crumbs { font-size: 14px; color: var(--muted); margin-top: 16px; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(220px, 1fr)); gap: 12px; }
.card { display: flex; flex-direction: column; gap: 4px; padding: 16px; background: var(--card);
  border: 1px solid var(--line); border-radius: 14px; text-decoration: none; color: var(--text); transition: transform .15s, border-color .15s; }
a.card:hover { transform: translateY(-2px); border-color: var(--accent); }
.card span { color: var(--muted); font-size: 14px; }
.card.off { opacity: .55; }
.chapter ul { padding-left: 20px; }
.btn { background: var(--accent); color: #fff; border: 0; border-radius: 10px; padding: 10px 16px; font: inherit; cursor: pointer; }
.btn:focus-visible, a:focus-visible, input:focus-visible { outline: 3px solid var(--accent-2); outline-offset: 2px; }
.tools { margin: 10px 0; }
.anim { background: var(--card); border: 1px solid var(--line); border-radius: 16px; padding: 12px; margin: 16px 0; }
#anim { min-height: 320px; }
#anim canvas, #anim svg { width: 100%; display: block; border-radius: 10px; }
.ctrl { display: flex; flex-wrap: wrap; gap: 12px 20px; align-items: center; margin-top: 10px; font-size: 15px; }
.ctrl label { display: flex; align-items: center; gap: 8px; }
.ctrl select, .ctrl input[type=range] { accent-color: var(--accent); }
.readout { font-variant-numeric: tabular-nums; font-weight: 600; }
.formulas { background: var(--code); border-radius: 12px; padding: 14px 14px 14px 34px; font-family: ui-monospace, Consolas, monospace; font-size: 15.5px; }
.ex { background: var(--card); border: 1px solid var(--line); border-radius: 12px; padding: 12px 14px; margin: 10px 0; }
.ex p { margin: 6px 0 0; }
.quiz .q { border: 1px solid var(--line); border-radius: 12px; margin: 0 0 10px; padding: 10px 14px; background: var(--card); }
.quiz legend { font-weight: 600; padding: 0 4px; }
.quiz label { display: block; padding: 3px 0; cursor: pointer; }
.quiz .right { border-color: var(--ok); }
.quiz .wrong { border-color: var(--bad); }
.quiz output { margin-left: 12px; font-weight: 700; }
details { background: var(--card); border: 1px solid var(--line); border-radius: 12px; padding: 10px 14px; margin: 8px 0; }
summary { cursor: pointer; }
.taught { width: 100%; border-collapse: collapse; font-size: 15px; }
.taught td { border-bottom: 1px solid var(--line); padding: 8px 6px; }
.foot { margin-top: 60px; background: var(--card); border-top: 1px solid var(--line); padding: 10px 0 30px; }
.foot h2 { font-size: 1.05rem; }
.sisters { list-style: none; padding: 0; display: flex; flex-wrap: wrap; gap: 8px 18px; }
.note { color: var(--muted); font-size: 14px; }
@media (prefers-reduced-motion: reduce) { * { transition: none !important; } }

/* ---------- motion system (from the motion kit: one easing language, transform + opacity only) ---------- */
:root {
  --ease-enter: cubic-bezier(.16, 1, .3, 1); --ease-std: cubic-bezier(.4, 0, .2, 1);
  --t-fast: .18s; --t-base: .32s; --t-slow: .5s;
  --grad: linear-gradient(90deg, #3b82f6, #a855f7, #ec4899, #f59e0b, #22c55e, #3b82f6);
}
.btn { transition: transform var(--t-fast) var(--ease-std), filter var(--t-fast); }
.btn:active { transform: scale(.97); }
.btn:disabled { opacity: .45; cursor: default; }
.btn-soft { background: var(--code); color: var(--text); }
.btn-ghost { background: transparent; color: var(--text); border: 1px solid var(--line); }
/* glow CTA: spinning gradient border */
.btn-glow { position: relative; isolation: isolate; background: var(--accent); }
.btn-glow::before { content: ""; position: absolute; inset: -2px; z-index: -1; border-radius: 12px;
  background: var(--grad); background-size: 300% 100%; animation: glow 6s linear infinite; filter: blur(.5px); }
.btn-glow::after { content: ""; position: absolute; inset: 0; z-index: -1; border-radius: 10px; background: var(--accent); }
@keyframes glow { to { background-position: 300% 0; } }
@media (hover: hover) and (pointer: fine) { .btn:hover { filter: brightness(1.08); } a.card:hover { transform: translateY(-3px); } }

.reveal.rv { opacity: 0; transform: translateY(20px); }
.reveal.in { opacity: 1; transform: none; transition: opacity var(--t-slow) var(--ease-enter), transform var(--t-slow) var(--ease-enter); }

/* ---------- home hero ---------- */
.hero-lead { font-size: 1.15rem; max-width: 640px; }
.hero-cta { display: flex; align-items: center; gap: 14px; flex-wrap: wrap; margin: 6px 0 4px; }
.hero-cta .btn { text-decoration: none; padding: 12px 20px; font-weight: 600; }
.hint { color: var(--muted); font-size: 14px; }
.hero3d { margin: 8px -16px 0; }
.why { list-style: none; padding: 0; display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr)); gap: 10px; }
.why li { background: var(--card); border: 1px solid var(--line); border-radius: 14px; padding: 12px 14px; font-weight: 500; }
.subj h3 { display: block; font-size: 1.15rem; margin: 22px 0 10px; }
.lesson-card span { line-height: 1.45; }
.revise { background: var(--card); border: 2px dashed var(--accent-2); border-radius: 16px; padding: 4px 16px 14px; margin: 16px 0; }
.chips { display: flex; flex-wrap: wrap; gap: 8px; }
.chip { background: var(--code); border-radius: 999px; padding: 6px 14px; text-decoration: none; font-weight: 600; }

/* ---------- 3D stage + story ---------- */
.stage3d { position: relative; width: 100%; border-radius: 12px; overflow: hidden; touch-action: pan-y; cursor: grab;
  background: radial-gradient(ellipse at 50% 30%, color-mix(in srgb, var(--accent) 14%, transparent), transparent 70%); }
.stage3d.grab { cursor: grabbing; touch-action: none; }
.stage3d canvas { width: 100% !important; height: 100% !important; display: block; }
.nogl { padding: 40px 16px; text-align: center; color: var(--muted); }
#anim { min-height: 0; }
.drag-hint { color: var(--muted); font-size: 13px; margin: 8px 2px 0; }
.story { margin-top: 12px; }
.story-h { font-size: 1.1rem; margin: 6px 0; }
.story ol { padding-left: 22px; margin: 6px 0; }
.story.js ol { display: none; }  /* without JS every step stays readable as a list */
.story-now { font-size: 1.12rem; line-height: 1.55; min-height: 3.2em; margin: 8px 0 12px; padding: 12px 14px;
  background: var(--code); border-radius: 12px; border-left: 5px solid var(--accent); }
.story-now b { color: var(--accent); margin-right: 6px; }
.story-now.pop { animation: pop var(--t-base) var(--ease-enter); }
@keyframes pop { from { opacity: .3; transform: translateY(6px); } }
.story-bar { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.story-bar .btn { padding: 10px 14px; }
.dots { display: flex; gap: 6px; flex: 1; justify-content: center; min-width: 120px; }
.dots button { width: 12px; height: 12px; border-radius: 50%; border: 0; padding: 0; background: var(--line); cursor: pointer;
  transition: transform var(--t-fast) var(--ease-std), background var(--t-fast); }
.dots button.done { background: color-mix(in srgb, var(--accent) 50%, var(--line)); }
.dots button.on { background: var(--accent); transform: scale(1.35); }
.twin { display: grid; grid-template-columns: repeat(auto-fit, minmax(260px, 1fr)); gap: 12px; margin: 16px 0; }
.hook, .real { border-radius: 16px; padding: 4px 16px 10px; border: 1px solid var(--line); }
.hook { background: color-mix(in srgb, #f59e0b 13%, var(--card)); }
.real { background: color-mix(in srgb, #22c55e 12%, var(--card)); }
.hook h2, .real h2 { font-size: 1.05rem; margin: 12px 0 4px; }
.quiz label.correct { font-weight: 700; color: var(--ok); }
.quiz { position: relative; }
.confetti { position: absolute; left: 50%; bottom: 40px; pointer-events: none; }
.confetti i { position: absolute; width: 8px; height: 12px; border-radius: 2px; opacity: 0;
  animation: fly 1.4s var(--ease-enter) var(--d) forwards; }
@keyframes fly { 0% { opacity: 1; transform: translate(0, 0) rotate(0); }
  100% { opacity: 0; transform: translate(calc(var(--x) * 220px), -240px) rotate(540deg); } }
@media (max-width: 560px) {
  body { font-size: 16px; }
  .story-bar .btn-ghost { order: 4; width: 100%; }
}
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: .01ms !important; animation-iteration-count: 1 !important; transition-duration: .01ms !important; }
}