/* ============================================================
   Online Calculators Free — Tab Divider Manual design system
   World: boxed reference manuals & binder dividers.
   Every section a board hue at full strength; every calculator
   a milk-acetate card hinged over its board; a stepped tab rail
   down the fore edge; hairline seams, never shadows; all
   computed figures in monospaced tabular numerals; states
   stamp themselves; motion is 90ms two-frame steps, no easing.
   ============================================================ */

/* ---------- Fonts (self-hosted, latin) ---------- */
@font-face { font-family: 'Archivo'; font-style: normal; font-weight: 400; font-display: swap; src: url('../fonts/Archivo-400.woff2') format('woff2'); }
@font-face { font-family: 'Archivo'; font-style: normal; font-weight: 500; font-display: swap; src: url('../fonts/Archivo-500.woff2') format('woff2'); }
@font-face { font-family: 'Archivo'; font-style: normal; font-weight: 600; font-display: swap; src: url('../fonts/Archivo-600.woff2') format('woff2'); }
@font-face { font-family: 'Archivo'; font-style: normal; font-weight: 700; font-display: swap; src: url('../fonts/Archivo-700.woff2') format('woff2'); }
@font-face { font-family: 'Archivo'; font-style: normal; font-weight: 800; font-display: swap; src: url('../fonts/Archivo-800.woff2') format('woff2'); }
@font-face { font-family: 'Source Serif 4'; font-style: normal; font-weight: 400; font-display: swap; src: url('../fonts/SourceSerif4-400.woff2') format('woff2'); }
@font-face { font-family: 'Source Serif 4'; font-style: normal; font-weight: 600; font-display: swap; src: url('../fonts/SourceSerif4-600.woff2') format('woff2'); }
@font-face { font-family: 'Source Serif 4'; font-style: normal; font-weight: 700; font-display: swap; src: url('../fonts/SourceSerif4-700.woff2') format('woff2'); }
@font-face { font-family: 'Source Serif 4'; font-style: italic; font-weight: 400; font-display: swap; src: url('../fonts/SourceSerif4-400i.woff2') format('woff2'); }
@font-face { font-family: 'IBM Plex Mono'; font-style: normal; font-weight: 400; font-display: swap; src: url('../fonts/PlexMono-400.woff2') format('woff2'); }
@font-face { font-family: 'IBM Plex Mono'; font-style: normal; font-weight: 500; font-display: swap; src: url('../fonts/PlexMono-500.woff2') format('woff2'); }
@font-face { font-family: 'IBM Plex Mono'; font-style: normal; font-weight: 600; font-display: swap; src: url('../fonts/PlexMono-600.woff2') format('woff2'); }

/* ---------- Tokens ----------
   --board is set per page (inline on <html>) to the category hue.
   Reading fields are acetate: a whisper of the board over paper. */
:root {
  --paper: #F7F4ED;          /* milk acetate */
  --paper-2: #FFFDF8;        /* brighter card field */
  --ink: #201D18;            /* warm black */
  --ink-70: #4E4A42;
  --ink-50: #625C51;
  --rule: color-mix(in srgb, var(--ink) 16%, transparent);
  --rule-strong: color-mix(in srgb, var(--ink) 42%, transparent);
  --board: #3D4FB7;          /* default board; overridden per category */
  --board-ink: #FFFDF8;      /* ink that reads on the board */
  --board-deep: color-mix(in srgb, var(--board) 72%, #14120E);
  --board-tint: color-mix(in srgb, var(--board) 9%, var(--paper));
  --board-tint-2: color-mix(in srgb, var(--board) 16%, var(--paper));
  --board-line: color-mix(in srgb, var(--board) 34%, transparent);
  --vermilion: #D0342C;      /* errata only */
  --ok: #2C6E49;
  --ground: var(--board-tint);
  --card: var(--paper-2);
  --focus: var(--board-deep);
  --mono: 'IBM Plex Mono', ui-monospace, 'Cascadia Mono', Consolas, monospace;
  --sans: 'Archivo', system-ui, -apple-system, 'Segoe UI', sans-serif;
  --serif: 'Source Serif 4', Georgia, 'Times New Roman', serif;
  --step: 90ms;              /* the world moves in two-frame steps */
}

[data-board-ink="dark"] { --board-ink: #171410; }

html.dark {
  --paper: #17151C;
  --paper-2: #1F1D25;
  --ink: #EFEAE0;
  --ink-70: #C4BFB4;
  --ink-50: #9A958B;
  --rule: color-mix(in srgb, var(--ink) 18%, transparent);
  --rule-strong: color-mix(in srgb, var(--ink) 44%, transparent);
  --board-deep: color-mix(in srgb, var(--board) 58%, #0E0D12);
  --board-tint: color-mix(in srgb, var(--board) 16%, #141218);
  --board-tint-2: color-mix(in srgb, var(--board) 24%, #141218);
  --board-line: color-mix(in srgb, var(--board) 45%, transparent);
  --card: var(--paper-2);
  --focus: color-mix(in srgb, var(--board) 62%, var(--ink));
}

/* ---------- Reset & base ---------- */
*, *::before, *::after { box-sizing: border-box; }
[hidden] { display: none !important; }
html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; }
@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } }
body {
  margin: 0;
  font-family: var(--serif);
  font-size: 17px;
  line-height: 1.62;
  color: var(--ink);
  background: var(--ground);
  transition: background-color var(--step) steps(2), color var(--step) steps(2);
}
img, svg { display: block; max-width: 100%; }
a { color: var(--board-deep); text-decoration-thickness: 1px; text-underline-offset: 3px; }
a:hover { text-decoration-thickness: 2px; }
html.dark a { color: color-mix(in srgb, var(--board) 55%, var(--ink)); }
:focus-visible { outline: 2px solid var(--focus); outline-offset: 2px; }
::selection { background: var(--board); color: var(--board-ink); }
input, select, textarea { caret-color: var(--board-deep); }
html.dark input, html.dark select, html.dark textarea { caret-color: color-mix(in srgb, var(--board) 60%, var(--ink)); }
* { scrollbar-width: thin; scrollbar-color: var(--rule-strong) transparent; }
::-webkit-scrollbar { width: 10px; height: 10px; }
::-webkit-scrollbar-thumb { background: var(--rule-strong); border: 2px solid transparent; background-clip: content-box; }
::-webkit-scrollbar-thumb:hover { background: var(--ink-50); border: 2px solid transparent; background-clip: content-box; }
::-webkit-scrollbar-track { background: transparent; }
h1, h2, h3, h4 { font-family: var(--sans); line-height: 1.15; margin: 0; font-weight: 700; }
p { margin: 0 0 1em; }
figure { margin: 0; }

.skip {
  position: absolute; left: -9999px; top: 0; z-index: 200;
  background: var(--ink); color: var(--paper); padding: .6rem 1rem;
  font-family: var(--sans); font-size: .85rem;
}
.skip:focus { left: 0; }

.wrap { max-width: 1200px; margin: 0 auto; padding: 0 clamp(1rem, 3vw, 2rem); }

/* ---------- Header ---------- */
.masthead {
  border-bottom: 1.5px solid var(--rule-strong);
  background: var(--paper);
  position: sticky; top: 0; z-index: 60;
}
html.dark .masthead { background: var(--paper); }
.masthead-inner {
  display: flex; align-items: center; gap: 1rem;
  padding-top: .65rem; padding-bottom: .65rem;
}
@media (max-width: 560px) {
  .masthead .masthead-inner { gap: .65rem; }
  .masthead .header-search kbd { display: none; }
  .masthead .brand-name { font-size: .92rem; max-width: 7.6rem; }
  .masthead .brand-name small { display: none; }
  .masthead .header-search input { padding: .52rem .6rem .52rem 2.15rem; }
  .masthead .icon-btn { padding: .45rem .5rem; }
  .masthead .icon-btn span { display: none; }
}
.brand { display: flex; align-items: center; gap: .55rem; text-decoration: none; color: var(--ink); flex-shrink: 0; }
.brand-mark { width: 34px; height: 34px; flex-shrink: 0; }
.brand-name { font-family: var(--sans); font-weight: 800; font-size: 1.02rem; line-height: 1.08; letter-spacing: -0.015em; max-width: 9.2rem; text-wrap: balance; }
.brand-name small { display: block; font-weight: 500; font-size: .69rem; letter-spacing: .18em; text-transform: uppercase; color: var(--ink-50); line-height: 1; margin-top: 3px; }

.header-search { flex: 1; min-width: 0; max-width: 560px; margin-left: auto; position: relative; }
.header-search input { min-width: 0; width: 100%; }
.header-search input {
  width: 100%; font: 500 .92rem/1.4 var(--sans);
  padding: .58rem 2.6rem .58rem 2.4rem;
  border: 1.5px solid var(--rule-strong); border-radius: 0;
  background: var(--card); color: var(--ink);
}
.header-search input:focus { border-color: var(--board); outline: none; box-shadow: inset 0 -2px 0 var(--board); }
.header-search .s-icon { position: absolute; left: .7rem; top: 50%; translate: 0 -50%; width: 15px; height: 15px; color: var(--ink-50); }
.header-search kbd {
  position: absolute; right: .5rem; top: 50%; translate: 0 -50%;
  font: 600 .69rem var(--mono); color: var(--ink-50);
  border: 1px solid var(--rule); padding: .1rem .35rem; border-radius: 2px;
}
.search-drop {
  position: absolute; top: calc(100% + 4px); left: 0; right: 0; z-index: 80;
  background: var(--card); border: 1.5px solid var(--rule-strong);
  max-height: 420px; overflow: auto; display: none;
}
.search-drop.open { display: block; }
.search-drop a {
  display: flex; justify-content: space-between; gap: .8rem; align-items: baseline;
  padding: .55rem .8rem; text-decoration: none; color: var(--ink);
  border-bottom: 1px solid var(--rule); font-family: var(--sans); font-size: .92rem;
}
.search-drop a:last-child { border-bottom: 0; }
.search-drop a:hover, .search-drop a.active { background: var(--board-tint-2); }
.search-drop .cat { font: 500 .69rem var(--mono); letter-spacing: .06em; text-transform: uppercase; color: var(--ink-50); white-space: nowrap; }
.search-drop .no-hit { padding: .7rem .8rem; font: 400 .9rem var(--sans); color: var(--ink-50); }

.header-actions { display: flex; gap: .4rem; align-items: center; flex-shrink: 0; }
.icon-btn {
  display: inline-flex; align-items: center; gap: .4rem;
  font: 600 .8rem var(--sans); color: var(--ink);
  border: 1.5px solid var(--rule-strong); background: var(--card);
  padding: .45rem .7rem; cursor: pointer; text-decoration: none;
  transition: background-color var(--step) steps(2);
}
.icon-btn:hover { background: var(--board-tint-2); }
.icon-btn svg { width: 15px; height: 15px; }
.fav-btn .n { font: 600 .72rem var(--mono); color: var(--board-deep); }

/* ---------- Stepped tab rail (the signature) ---------- */
.rail {
  position: sticky; top: 0; z-index: 50;
  background: var(--ground);
  border-bottom: 1.5px solid var(--rule-strong);
}
.rail-inner { display: flex; align-items: stretch; gap: 0; overflow-x: auto; scrollbar-width: thin; padding: 0 clamp(1rem, 3vw, 2rem); max-width: 1200px; margin: 0 auto; }
@media (max-width: 899.98px) {
  .rail { -webkit-mask-image: linear-gradient(to right, #000 86%, transparent); mask-image: linear-gradient(to right, #000 86%, transparent); }
}
@media (min-width: 900px) {
  .rail-inner { flex-wrap: wrap; overflow-x: visible; }
}
.rail-label {
  font: 700 .69rem var(--mono); letter-spacing: .2em; text-transform: uppercase;
  color: var(--ink-50); align-self: center; padding-right: .9rem; flex-shrink: 0;
  border-right: 1px solid var(--rule); margin-right: .4rem; writing-mode: horizontal-tb;
}
.tab {
  position: relative; display: inline-flex; align-items: center;
  font: 600 .78rem/1 var(--sans); letter-spacing: .02em;
  color: var(--ink); text-decoration: none; white-space: nowrap;
  padding: .69rem .78rem .6rem calc(.78rem + var(--step-i, 0) * 1px);
  border-bottom: 3px solid transparent; margin-bottom: -1.5px;
  transition: background-color var(--step) steps(2);
}
.tab::before {
  content: ''; position: absolute; left: calc(.3rem + var(--step-i, 0) * 1px); top: 30%;
  width: 5px; height: 5px; border-radius: 50%;
  background: var(--tc, var(--board)); opacity: .85;
}
.tab:hover { background: color-mix(in srgb, var(--tc, var(--board)) 14%, transparent); }
.tab.active { border-bottom-color: var(--tc, var(--board)); font-weight: 700; }
.tab.active::before { width: 8px; height: 8px; top: 27%; opacity: 1; }
.tab.more { font-weight: 500; color: var(--ink-50); }

/* Vertical rail on wide calculator/category pages */
.rail-v { display: none; }
@media (min-width: 1100px) {
  /* the vertical rail replaces the horizontal one: one Sections nav in the
     page and in the accessibility tree (a11y audit 2026-11-01) */
  .rail { display: none; }
  .with-rail { display: grid; grid-template-columns: 218px minmax(0, 1fr); gap: 0; }
  .with-rail .rail-v {
    display: block; position: sticky; top: 62px; align-self: start;
    padding: 1.4rem 0 2rem; border-right: 1.5px solid var(--rule-strong);
    margin-right: clamp(1.2rem, 3vw, 2.4rem);
    max-height: calc(100vh - 62px); overflow-y: auto; scrollbar-width: thin;
  }
  .rail-v .rail-title {
    font: 700 .69rem var(--mono); letter-spacing: .22em; text-transform: uppercase;
    color: var(--ink-50); margin: 0 0 .9rem; padding-left: 1.1rem;
  }
  .rail-v .vtab {
    display: flex; align-items: center; gap: .55rem;
    font: 600 .8rem/1.2 var(--sans); color: var(--ink); text-decoration: none;
    padding: .5rem .6rem .5rem .55rem;
    border-left: 4px solid transparent;
    margin: 1px 0 1px calc(var(--step-i, 0) * 4px);
    transition: background-color var(--step) steps(2);
  }
  .rail-v .vtab .chip { width: 10px; height: 10px; flex-shrink: 0; background: var(--tc); }
  .rail-v .vtab:hover { background: color-mix(in srgb, var(--tc) 13%, transparent); }
  .rail-v .vtab.active { border-left-color: var(--tc); font-weight: 700; background: color-mix(in srgb, var(--tc) 10%, transparent); }
  .rail-v .vtab.active .chip { outline: 1.5px solid var(--tc); outline-offset: 2px; }
}
@media (max-width: 1099px) { .with-rail { display: block; } }

/* ---------- Hero (home) ---------- */
.hero { border-bottom: 1.5px solid var(--rule-strong); background: var(--paper); position: relative; }
.hero-grid { display: grid; grid-template-columns: minmax(0, 7fr) minmax(0, 5fr); gap: 2.5rem; padding-top: 3.2rem; padding-bottom: 3rem; align-items: center; }
@media (max-width: 900px) { .hero-grid { grid-template-columns: minmax(0, 1fr); padding-top: 2.4rem; padding-bottom: 2rem; } }
.hero h1 {
  font-size: clamp(2.1rem, 4.6vw, 3.4rem); font-weight: 800; letter-spacing: -0.025em;
  margin: .5rem 0 1rem; max-width: 14ch;
}
.hero h1 em { font-style: normal; box-shadow: inset 0 -0.28em var(--board-tint-2); }
.hero p.lede { font-size: 1.12rem; color: var(--ink-70); max-width: 46ch; }
.hero-search { margin-top: 1.4rem; position: relative; max-width: 520px; }
.hero-search input {
  width: 100%; font: 500 1.02rem/1.4 var(--sans); padding: .85rem 1rem .85rem 2.7rem;
  border: 2px solid var(--ink); background: var(--card); color: var(--ink);
}
.hero-search input:focus { outline: 3px solid var(--board); outline-offset: 2px; }
.hero-search .s-icon { position: absolute; left: .85rem; top: 50%; translate: 0 -50%; width: 17px; height: 17px; color: var(--ink-50); }
.hero-stats { display: flex; gap: 1.6rem; margin-top: 1.6rem; flex-wrap: wrap; }
.hero-stats div { font-family: var(--sans); }
.hero-stats b { display: block; font: 700 1.45rem var(--mono); color: var(--board-deep); letter-spacing: -0.02em; }
html.dark .hero-stats b { color: color-mix(in srgb, var(--board) 58%, var(--ink)); }
.hero-stats span { font-size: .76rem; letter-spacing: .1em; text-transform: uppercase; color: var(--ink-50); }

/* Hero side: the divider stack — 14 boards, stepped */
.divider-stack { display: flex; flex-direction: column; gap: 7px; padding-left: clamp(0px, 2vw, 14px); min-width: 0; }
.divider-stack a {
  display: flex; align-items: center; justify-content: space-between; gap: 1rem;
  padding: .52rem .9rem .52rem .8rem;
  margin-left: calc(var(--step-i, 0) * 7px);
  text-decoration: none; background: var(--tc); color: var(--ti, #FFFDF8);
  font: 700 .88rem var(--sans); letter-spacing: .01em;
  border: 1.5px solid color-mix(in srgb, var(--tc) 62%, #14120E);
  transition: padding var(--step) steps(2), translate var(--step) steps(2);
}
.divider-stack a:hover { translate: 8px 0; }
.divider-stack a span.n { font: 600 .69rem var(--mono); }

/* ---------- Section headers ---------- */
.section { padding: 2.6rem 0; }
.section-head { display: flex; align-items: baseline; gap: 1rem; margin-bottom: 1.4rem; border-bottom: 2px solid var(--ink); padding-bottom: .5rem; }
.section-head h2 { font-size: clamp(1.3rem, 2.4vw, 1.7rem); letter-spacing: -0.015em; }
.section-head .count { font: 600 .75rem var(--mono); color: var(--ink-50); margin-left: auto; letter-spacing: .08em; }
.section-head .more-link { font: 600 .82rem var(--sans); white-space: nowrap; }

/* ---------- Divider cards grid (categories) ---------- */
.board-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(255px, 1fr)); gap: 14px; }
.board-card { border: 1.5px solid var(--rule-strong); background: var(--card); display: flex; flex-direction: column; text-decoration: none; color: var(--ink); transition: translate var(--step) steps(2); }
.board-card:hover { translate: 0 -3px; }
.board-card .board-strip { height: 46px; background: var(--tc); color: var(--ti, #FFFDF8); display: flex; align-items: center; justify-content: space-between; padding: 0 .85rem; font: 700 .92rem var(--sans); letter-spacing: .01em; }
.board-card .board-strip .n { font: 600 .72rem var(--mono); }
.board-card .board-body { padding: .85rem .9rem 1rem; }
.board-card ul { margin: 0; padding: 0; list-style: none; }
.board-card li { font: 500 .88rem/1.9 var(--sans); border-bottom: 1px solid var(--rule); }
.board-card li:last-child { border-bottom: 0; }
.board-card li a { color: var(--ink); text-decoration: none; display: flex; justify-content: space-between; gap: .5rem; }
.board-card li a::after { content: '\2192'; color: var(--ink-50); font-family: var(--mono); font-size: .8em; }
.board-card li a:hover { color: var(--board-deep); }

/* ---------- Calculator page ---------- */
.crumbs { font: 500 .78rem var(--sans); color: var(--ink-50); padding: 1.1rem 0 .2rem; display: flex; gap: .45rem; flex-wrap: wrap; align-items: center; }
.crumbs a { color: var(--ink-70); text-decoration: none; }
.crumbs a:hover { text-decoration: underline; }
.crumbs .sep { font-family: var(--mono); }
.page-head { padding: .4rem 0 1.2rem; }
.page-head h1 { font-size: clamp(1.5rem, 3vw, 2.15rem); letter-spacing: -0.02em; margin: .1rem 0 .45rem; }
.page-head .desc { font-size: 1.02rem; color: var(--ink-70); max-width: 62ch; }
.page-head .meta { display: flex; gap: 1.1rem; margin-top: .7rem; font: 500 .72rem var(--mono); letter-spacing: .06em; text-transform: uppercase; color: var(--ink-50); flex-wrap: wrap; }

/* The Card: acetate over board */
.calc-card {
  border: 2px solid var(--ink);
  background: var(--card);
  display: grid; grid-template-columns: minmax(0, 7fr) minmax(0, 5fr);
  position: relative;
}
@media (max-width: 860px) { .calc-card { grid-template-columns: 1fr; } }
.calc-card::before, .calc-card::after {  /* punch holes on the bound edge */
  content: ''; position: absolute; left: 10px; width: 12px; height: 12px;
  border-radius: 50%; background: var(--ground); border: 1.5px solid var(--rule-strong);
}
.calc-card::before { top: 16px; } .calc-card::after { top: 44px; }
.calc-form { padding: 1.6rem 1.7rem 1.7rem 2.4rem; }
@media (max-width: 860px) { .calc-form { padding: 1.3rem 1.2rem 1.4rem 2.1rem; } }
.calc-form-title {
  font: 800 .8rem var(--sans); letter-spacing: .16em; text-transform: uppercase;
  border-bottom: 2px solid var(--ink); padding-bottom: .5rem; margin-bottom: 1.2rem;
  display: flex; justify-content: space-between; align-items: baseline; gap: 1rem;
}
.calc-form-title .fno { font: 600 .72rem var(--mono); color: var(--ink-50); letter-spacing: .08em; }

.field { margin-bottom: 1.05rem; }
.field > label { display: flex; justify-content: space-between; align-items: baseline; gap: .8rem; font: 600 .82rem var(--sans); margin-bottom: .35rem; }
.field .hint { font: 400 .72rem var(--sans); color: var(--ink-50); text-align: right; }
.field input[type="text"], .field input[type="number"], .field input[type="date"], .field select, .field input[type="time"] {
  width: 100%; font: 500 .98rem var(--mono); padding: .55rem .65rem;
  border: 1.5px solid var(--rule-strong); border-radius: 0; background: var(--paper-2); color: var(--ink);
  font-variant-numeric: tabular-nums;
}
.field input:focus, .field select:focus { border-color: var(--board); outline: none; box-shadow: inset 0 -2px 0 var(--board); }
.field input[aria-invalid="true"], .field select[aria-invalid="true"] { border-color: var(--vermilion); box-shadow: inset 0 -2px 0 var(--vermilion); }
.field .unit-wrap { display: flex; }
.field .unit-wrap input { flex: 1; min-width: 0; }
.field .unit-wrap .unit {
  font: 600 .78rem var(--mono); padding: .55rem .6rem; border: 1.5px solid var(--rule-strong); border-left: 0;
  color: var(--ink-70); background: var(--board-tint); display: flex; align-items: center; white-space: nowrap;
}
.field .err { font: 500 .74rem var(--sans); color: var(--vermilion); margin-top: .3rem; display: none; }
.field.invalid .err { display: block; }

.field-row { display: grid; grid-template-columns: 1fr 1fr; gap: .9rem; }
@media (max-width: 560px) { .field-row { grid-template-columns: 1fr; } }

.seg { display: flex; border: 1.5px solid var(--rule-strong); width: max-content; max-width: 100%; }
.seg label { font: 600 .8rem var(--sans); padding: .5rem .95rem; cursor: pointer; color: var(--ink-70); border-right: 1.5px solid var(--rule-strong); }
.seg label:last-child { border-right: 0; }
.seg input { position: absolute; opacity: 0; pointer-events: none; }
.seg input:checked + span { color: var(--board-ink); }
.seg label:has(input:checked) { background: var(--board); color: var(--board-ink); }
.seg label:has(input:focus-visible) { outline: 2px solid var(--focus); outline-offset: -2px; }

.presets { display: flex; flex-wrap: wrap; gap: .45rem; margin: .35rem 0 1rem; }
.presets .plabel { width: 100%; font: 700 .69rem var(--mono); letter-spacing: .16em; text-transform: uppercase; color: var(--ink-50); margin-bottom: .1rem; }
.preset {
  font: 600 .74rem var(--sans); padding: .3rem .6rem; cursor: pointer;
  border: 1px solid var(--rule-strong); background: var(--paper-2); color: var(--ink-70);
  transition: background-color var(--step) steps(2);
}
.preset:hover { background: var(--board-tint-2); color: var(--ink); }

.calc-actions { display: flex; gap: .7rem; margin-top: 1.3rem; align-items: center; flex-wrap: wrap; }
.btn {
  font: 700 .88rem var(--sans); letter-spacing: .04em; cursor: pointer;
  padding: .72rem 1.5rem; border: 2px solid var(--ink); background: var(--ink); color: var(--paper);
  transition: background-color var(--step) steps(2), color var(--step) steps(2);
}
.btn:hover { background: var(--board-deep); border-color: var(--board-deep); }
html.dark .btn:hover { background: var(--board); border-color: var(--board); color: var(--board-ink); }
.btn-ghost { background: transparent; color: var(--ink); border-color: var(--rule-strong); padding: .66rem 1.1rem; }
.btn-ghost:hover { background: var(--board-tint-2); color: var(--ink); border-color: var(--ink); }
.btn-copy { margin-left: auto; }

/* Results panel — the board */
.calc-result {
  background: var(--board); color: var(--board-ink);
  padding: 1.6rem 1.7rem 1.7rem; position: relative; overflow: hidden;
}
.calc-result .res-label { font: 700 .69rem var(--mono); letter-spacing: .22em; text-transform: uppercase; }
.res-main { margin: .9rem 0 1.1rem; }
.res-main .big {
  font: 600 clamp(2.6rem, 6vw, 4.1rem)/1 var(--mono); letter-spacing: -0.03em;
  font-variant-numeric: tabular-nums; display: block; word-break: break-all;
}
.res-main .big .u { font-size: .38em; letter-spacing: 0; opacity: .8; margin-left: .2em; }
.res-main .sub { font: 600 .95rem var(--sans); margin-top: .5rem; }
.res-row {
  display: flex; justify-content: space-between; gap: 1rem; align-items: baseline;
  font: 500 .84rem/1.5 var(--sans); padding: .5rem 0; border-top: 1.5px solid color-mix(in srgb, currentColor 26%, transparent);
}
.res-row b { font: 600 .92rem var(--mono); font-variant-numeric: tabular-nums; white-space: nowrap; }
.res-note { font: italic 400 .84rem/1.5 var(--serif); margin-top: .8rem; }

/* Split-flap result tick */
.flap { display: inline-block; overflow: hidden; vertical-align: bottom; }
.flap .cur, .flap .new { display: block; }
.flap .new { position: absolute; visibility: hidden; }
.flap.tick { animation: flap var(--step) steps(2); }
@keyframes flap { 0% { clip-path: inset(0 0 50% 0); } 100% { clip-path: inset(0 0 0 0); } }
@media (prefers-reduced-motion: reduce) { .flap.tick { animation: none; } }

/* Stamps */
.stamp {
  position: absolute; top: .9rem; right: .9rem; z-index: 5;
  font: 800 .69rem var(--sans); letter-spacing: .24em; text-transform: uppercase;
  color: var(--board-ink); border: 2.5px solid currentColor; border-radius: 3px;
  padding: .3rem .55rem; rotate: -6deg; opacity: 0; pointer-events: none;
  mix-blend-mode: normal; background: color-mix(in srgb, var(--board-ink) 6%, transparent);
}
.stamp.on { opacity: .94; transition: opacity var(--step) steps(2); }
.stamp.err { color: var(--vermilion); }

/* Formula block (errata card) */
.formula-card {
  margin-top: 1.2rem; border: 1.5px solid var(--rule-strong); background: var(--paper);
}
.formula-card header { font: 700 .69rem var(--mono); letter-spacing: .2em; text-transform: uppercase; padding: .55rem .9rem; border-bottom: 1px solid var(--rule-strong); color: var(--ink-70); display: flex; justify-content: space-between; }
.formula-card .fbody { padding: .85rem .95rem; font: 500 .95rem/1.75 var(--mono); font-variant-numeric: tabular-nums; overflow-x: auto; }
.formula-card .fsub { color: var(--ink-70); }
.formula-card .verr { color: var(--vermilion); font-family: var(--sans); font-weight: 600; }

/* Bars / spectrum (BMI gauge etc.) */
.spectrum { margin-top: 1rem; }
.spectrum .bar { display: flex; height: 12px; border: 1.5px solid color-mix(in srgb, currentColor 45%, transparent); }
.spectrum .bar i { flex: 1; }
.spectrum .ticks { display: flex; justify-content: space-between; font: 500 .64rem var(--mono); opacity: .8; margin-top: .25rem; }
.spectrum .marker { font: 600 .72rem var(--sans); margin-top: .4rem; display: flex; align-items: center; gap: .45rem; }
.spectrum .marker::before { content: ''; width: 9px; height: 9px; background: currentColor; border: 2px solid var(--paper); outline: 1.5px solid currentColor; }

/* Data table (schedules at true density) */
.tbl-wrap { overflow-x: auto; border: 1.5px solid var(--rule-strong); background: var(--card); margin-top: 1rem; }
table.data { border-collapse: collapse; width: 100%; font: 500 .8rem/1.5 var(--mono); font-variant-numeric: tabular-nums; }
table.data th { font: 700 .69rem var(--sans); letter-spacing: .1em; text-transform: uppercase; text-align: left; padding: .55rem .7rem; border-bottom: 2px solid var(--ink); background: var(--board-tint); position: sticky; top: 0; }
table.data td { padding: .42rem .7rem; border-bottom: 1px solid var(--rule); }
table.data tr:nth-child(even) td { background: color-mix(in srgb, var(--board) 4%, transparent); }
table.data td.num, table.data th.num { text-align: right; }

/* Content sections */
.prose { max-width: 72ch; }
.prose h2 { font-size: 1.28rem; margin: 2.1rem 0 .7rem; letter-spacing: -0.01em; }
.prose h2::before { content: ''; display: block; width: 34px; border-top: 3px solid var(--board); margin-bottom: .55rem; }
.prose p, .prose li { color: var(--ink-70); font-size: 1rem; }
.prose strong { color: var(--ink); }
.prose code { font: 500 .88em var(--mono); background: var(--board-tint-2); padding: .08em .35em; border-radius: 2px; }
.prose ul, .prose ol { padding-left: 1.3rem; margin: 0 0 1.2rem; }
.prose li { margin-bottom: .4rem; }
.margin-note {
  border: 1px solid var(--board-line); background: var(--board-tint);
  padding: .75rem 1rem; font: italic 400 .92rem/1.6 var(--serif); color: var(--ink-70);
  margin: 1.2rem 0; max-width: 60ch;
}
.margin-note b { font-style: normal; font-family: var(--sans); font-size: .72rem; letter-spacing: .14em; text-transform: uppercase; color: var(--board-deep); display: block; margin-bottom: .2rem; }
html.dark .margin-note b { color: color-mix(in srgb, var(--board) 60%, var(--ink)); }

/* Example / steps two-up */
.duo { display: grid; grid-template-columns: 1fr 1fr; gap: 1.3rem; margin: 1.2rem 0; }
@media (max-width: 760px) { .duo { grid-template-columns: 1fr; } }
.duo .box { border: 1.5px solid var(--rule-strong); background: var(--card); padding: 1rem 1.1rem; }
.duo .box h3 { font-size: .78rem; letter-spacing: .16em; text-transform: uppercase; margin-bottom: .6rem; color: var(--ink-70); }
.duo ol { margin: 0; padding-left: 1.15rem; }
.duo ol li { font: 500 .92rem/1.75 var(--sans); color: var(--ink-70); }

/* FAQ */
.faq details { border-bottom: 1px solid var(--rule-strong); background: var(--card); }
.faq details:first-child { border-top: 1.5px solid var(--rule-strong); }
.faq summary {
  cursor: pointer; list-style: none; padding: .95rem 1rem;
  font: 600 .98rem/1.4 var(--sans); display: flex; justify-content: space-between; gap: 1rem; align-items: baseline;
}
.faq summary::-webkit-details-marker { display: none; }
.faq summary::after { content: '+'; font: 600 1.2rem var(--mono); color: var(--board-deep); }
.faq details[open] summary::after { content: '\2013'; }
.faq details .fa { padding: 0 1rem 1rem; color: var(--ink-70); max-width: 68ch; }

/* Related grid */
.related { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 12px; }
.related a {
  text-decoration: none; color: var(--ink); border: 1.5px solid var(--rule-strong); background: var(--card);
  padding: .8rem .9rem; display: flex; flex-direction: column; gap: .3rem;
  transition: translate var(--step) steps(2);
}
.related a:hover { translate: 0 -3px; }
.related .rc { font: 700 .69rem var(--mono); letter-spacing: .18em; text-transform: uppercase; color: color-mix(in srgb, var(--tc, var(--board)) 55%, var(--ink)); }
.related .rn { font: 600 .92rem var(--sans); }
.related .rd { font: 400 .8rem/1.45 var(--sans); color: var(--ink-50); display: -webkit-box; -webkit-line-clamp: 2; -webkit-box-orient: vertical; overflow: hidden; }

/* A-Z directory */
.az-index { display: grid; grid-template-columns: repeat(auto-fill, minmax(240px, 1fr)); gap: 0 2rem; }
.az-letter { font: 800 1.7rem var(--sans); color: var(--board-deep); border-bottom: 2.5px solid var(--board); margin: 1.6rem 0 .6rem; }
.az-list { list-style: none; margin: 0; padding: 0; column-gap: 2rem; }
.az-list li { border-bottom: 1px solid var(--rule); }
.az-list a { display: flex; justify-content: space-between; gap: .6rem; padding: .42rem .15rem; font: 500 .9rem var(--sans); color: var(--ink); text-decoration: none; }
.az-list a:hover { color: var(--board-deep); text-decoration: underline; }
.az-list .cat { font: 500 .69rem var(--mono); color: var(--ink-50); white-space: nowrap; }

/* Category page directory */
.cat-list { list-style: none; padding: 0; margin: 0; border-top: 1.5px solid var(--rule-strong); }
.cat-list li { border-bottom: 1px solid var(--rule-strong); background: var(--card); }
.cat-list li:hover { background: var(--board-tint-2); }
.cat-list a { display: flex; align-items: baseline; gap: .8rem; padding: .69rem .9rem; text-decoration: none; color: var(--ink); }
.cat-list .cn { font: 600 .95rem var(--sans); flex: 1; }
.cat-list .cd { font: 400 .82rem var(--sans); color: var(--ink-50); flex: 2; display: -webkit-box; -webkit-line-clamp: 1; -webkit-box-orient: vertical; overflow: hidden; }
.cat-list .arr { font-family: var(--mono); color: var(--ink-50); }

/* Favorites/recents shelf */
.shelf { display: none; }
.shelf.has-items { display: block; border: 1.5px dashed var(--rule-strong); background: color-mix(in srgb, var(--card) 60%, transparent); padding: .8rem .95rem; margin: 0 0 1.4rem; }
.shelf .sh-head { display: flex; align-items: baseline; gap: .8rem; margin-bottom: .45rem; }
.shelf .sh-head b { font: 700 .69rem var(--mono); letter-spacing: .18em; text-transform: uppercase; color: var(--ink-70); }
.shelf .sh-head .clear { margin-left: auto; font: 600 .72rem var(--sans); color: var(--vermilion); background: none; border: none; cursor: pointer; text-decoration: underline; }
.shelf .items { display: flex; flex-wrap: wrap; gap: .45rem; }
.shelf .items a { font: 600 .78rem var(--sans); padding: .32rem .65rem; border: 1px solid var(--rule-strong); background: var(--card); color: var(--ink); text-decoration: none; }
.shelf .items a:hover { background: var(--board-tint-2); }

/* Footer */
.colophon { border-top: 2px solid var(--ink); margin-top: 3rem; background: var(--paper); }
.colophon .cols { display: grid; grid-template-columns: 2fr 1fr 1fr 1fr; gap: 2rem; padding: 2.2rem 0 1.6rem; }
@media (max-width: 860px) { .colophon .cols { grid-template-columns: 1fr 1fr; } }
.colophon h3 { font: 700 .69rem var(--mono); letter-spacing: .2em; text-transform: uppercase; color: var(--ink-50); margin-bottom: .7rem; }
.colophon ul { list-style: none; margin: 0; padding: 0; }
.colophon li { margin-bottom: .38rem; }
.colophon a { font: 500 .85rem var(--sans); color: var(--ink-70); text-decoration: none; }
.colophon a:hover { color: var(--ink); text-decoration: underline; }
.colophon .about { font: 400 .9rem/1.6 var(--serif); color: var(--ink-70); max-width: 34ch; }
.colophon .legal { border-top: 1px solid var(--rule-strong); padding: .9rem 0; display: flex; justify-content: space-between; gap: 1rem; flex-wrap: wrap; font: 500 .74rem var(--mono); color: var(--ink-50); }

/* Toast */
.toast {
  position: fixed; bottom: 1.2rem; left: 50%; translate: -50% 0; z-index: 120;
  background: var(--ink); color: var(--paper); font: 600 .85rem var(--sans);
  padding: .7rem 1.2rem; opacity: 0; pointer-events: none;
  transition: opacity var(--step) steps(2);
}
.toast.on { opacity: 1; }

/* Utility */
.muted { color: var(--ink-50); }
.nowrap { white-space: nowrap; }

@media print {
  .masthead, .rail, .rail-v, .calc-actions, .colophon, .shelf, .related, .crumbs { display: none !important; }
  body { background: #fff; }
  .calc-card { border-width: 1px; }
}

/* coarse-pointer nudges */
@media (hover: none) {
  .tab { padding-top: .8rem; padding-bottom: .75rem; }
  .btn { padding-block: .85rem; }
}