/* Benchtop — chemistry. Light/dark tokens, then layout, then views. */
:root {
  color-scheme: light;
  --bg: #F1F4F6;
  --grid: rgba(30, 70, 100, .055);
  --surface: #FFFFFF;
  --surface-2: #F6F8FA;
  --surface-3: #ECF0F3;
  --ink: #15202B;
  --muted: #56646F;
  --faint: #8A96A1;
  --line: #DAE0E5;
  --line-strong: #C3CCD3;
  --accent: #0E7C66;
  --accent-ink: #FFFFFF;
  --accent-soft: #DDF1EA;
  --warn: #A96A12;
  --bad: #BD3A21;
  --bad-soft: #FAE5E0;
  --good: #0E7C66;
  --focus: #2563EB;
  --shadow: 0 1px 2px rgba(20, 32, 43, .06), 0 8px 28px rgba(20, 32, 43, .10);
  --scrim: rgba(13, 19, 24, .38);

  /* element categories */
  --c-alkali: #E5484D;
  --c-alkaline: #F76B15;
  --c-transition: #D39B00;
  --c-post: #30A46C;
  --c-metalloid: #12A594;
  --c-nonmetal: #0588F0;
  --c-halogen: #6E56CF;
  --c-noble: #D6409F;
  --c-lanthanide: #A07C3F;
  --c-actinide: #7A7F66;
  --tile-mix: 20%;

  /* particles */
  --proton: #E5484D;
  --neutron: #8FA3B5;
  --electron: #0588F0;
  --electron-gain: #F76B15;

  /* sequential ramp (low → high) */
  --ramp-0: #EEF6F3;
  --ramp-1: #B9E0D3;
  --ramp-2: #6FC0A8;
  --ramp-3: #2A927A;
  --ramp-4: #0B5B4B;

  --font-display: "Barlow Semi Condensed", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", "Noto Sans SC", system-ui, sans-serif;
  --font-body: "IBM Plex Sans", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", "Noto Sans SC", system-ui, -apple-system, "Segoe UI", sans-serif;
  --font-mono: "IBM Plex Mono", ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  --r: 10px;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    color-scheme: dark;
    --bg: #0D1318; --grid: rgba(150, 190, 220, .045); --surface: #141C23; --surface-2: #19232B; --surface-3: #1F2A33;
    --ink: #E3E9EE; --muted: #9BA8B3; --faint: #6C7985; --line: #26313B; --line-strong: #35424E;
    --accent: #3AC4A0; --accent-ink: #06231C; --accent-soft: #113229; --warn: #E3AE55; --bad: #F28A72; --bad-soft: #3A1D17; --good: #3AC4A0;
    --focus: #7FA8FF; --shadow: 0 1px 2px rgba(0, 0, 0, .3), 0 10px 32px rgba(0, 0, 0, .45); --scrim: rgba(0, 0, 0, .55);
    --c-alkali: #FF6369; --c-alkaline: #FF8B3E; --c-transition: #F0C000; --c-post: #3DD68C; --c-metalloid: #0BD8B6;
    --c-nonmetal: #3B9EFF; --c-halogen: #9B8AFB; --c-noble: #F76CC0; --c-lanthanide: #C9A46B; --c-actinide: #A6AC8E; --tile-mix: 24%;
    --proton: #FF6369; --neutron: #8093A5; --electron: #52A9FF; --electron-gain: #FF8B3E;
    --ramp-0: #15241F; --ramp-1: #164536; --ramp-2: #1F7A60; --ramp-3: #33B891; --ramp-4: #9EEBD2;
  }
}
:root[data-theme="dark"] {
  color-scheme: dark;
  --bg: #0D1318; --grid: rgba(150, 190, 220, .045); --surface: #141C23; --surface-2: #19232B; --surface-3: #1F2A33;
  --ink: #E3E9EE; --muted: #9BA8B3; --faint: #6C7985; --line: #26313B; --line-strong: #35424E;
  --accent: #3AC4A0; --accent-ink: #06231C; --accent-soft: #113229; --warn: #E3AE55; --bad: #F28A72; --bad-soft: #3A1D17; --good: #3AC4A0;
  --focus: #7FA8FF; --shadow: 0 1px 2px rgba(0, 0, 0, .3), 0 10px 32px rgba(0, 0, 0, .45); --scrim: rgba(0, 0, 0, .55);
  --c-alkali: #FF6369; --c-alkaline: #FF8B3E; --c-transition: #F0C000; --c-post: #3DD68C; --c-metalloid: #0BD8B6;
  --c-nonmetal: #3B9EFF; --c-halogen: #9B8AFB; --c-noble: #F76CC0; --c-lanthanide: #C9A46B; --c-actinide: #A6AC8E; --tile-mix: 24%;
  --proton: #FF6369; --neutron: #8093A5; --electron: #52A9FF; --electron-gain: #FF8B3E;
  --ramp-0: #15241F; --ramp-1: #164536; --ramp-2: #1F7A60; --ramp-3: #33B891; --ramp-4: #9EEBD2;
}

*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; scroll-padding-top: 80px; }
body {
  margin: 0; min-height: 100vh; color: var(--ink); font: 400 15px/1.55 var(--font-body);
  background-color: var(--bg);
  background-image: linear-gradient(var(--grid) 1px, transparent 1px), linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 24px 24px;
}
[hidden] { display: none !important; }
button, input, select, textarea { font: inherit; color: inherit; }
button { cursor: pointer; }
a { color: var(--accent); }
:focus-visible { outline: 2px solid var(--focus); outline-offset: 2px; }
sub, sup { font-size: .68em; line-height: 0; }
.mono { font-family: var(--font-mono); font-variant-numeric: tabular-nums; }
.skip { position: absolute; left: -999px; top: 8px; background: var(--surface); padding: 8px 12px; border-radius: 6px; z-index: 100; }
.skip:focus { left: 8px; }
.boot { padding: 48px 20px; text-align: center; color: var(--muted); }
.sr { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }

/* ---------- header ---------- */
.top { position: sticky; top: 0; z-index: 20; background: color-mix(in oklab, var(--bg) 88%, transparent); backdrop-filter: blur(10px); -webkit-backdrop-filter: blur(10px); border-bottom: 1px solid var(--line); padding-top: env(safe-area-inset-top, 0px); }
.top-inner { max-width: 1320px; margin: 0 auto; padding: 10px 20px; display: flex; align-items: center; gap: 12px 24px; flex-wrap: wrap; }
.brand { display: inline-flex; align-items: center; gap: 9px; text-decoration: none; color: var(--ink); }
.brand svg { width: 28px; height: 28px; flex: none; }
.brand svg rect { fill: var(--accent); }
.brand svg circle { fill: var(--accent-ink); }
.brand svg g { stroke: var(--accent-ink); }
.brand-name { font: 700 22px/1 var(--font-display); letter-spacing: -.01em; }
.brand-sub { font: 500 12px/1 var(--font-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--muted); padding-top: 3px; }
.tabs { display: flex; gap: 2px; flex: 1 1 auto; }
.tabs a { padding: 7px 12px; border-radius: 7px; text-decoration: none; color: var(--muted); font-weight: 500; font-size: 14.5px; white-space: nowrap; }
.tabs a:hover { color: var(--ink); background: var(--surface); }
.tabs a[aria-current="page"] { color: var(--ink); background: var(--surface); box-shadow: inset 0 0 0 1px var(--line); }
.controls { display: flex; gap: 8px; align-items: center; }
.seg { display: inline-flex; border: 1px solid var(--line); background: var(--surface); border-radius: 8px; padding: 2px; }
.seg button { border: 0; background: none; padding: 5px 10px; border-radius: 6px; font-size: 13px; font-weight: 600; color: var(--muted); line-height: 1.2; }
.seg button[aria-pressed="true"] { background: var(--ink); color: var(--bg); }
.icon-btn { width: 34px; height: 34px; display: inline-grid; place-items: center; border: 1px solid var(--line); background: var(--surface); border-radius: 8px; font-weight: 600; font-size: 14px; }
.icon-btn svg { width: 17px; height: 17px; fill: currentColor; }
.icon-btn:hover, .seg button:hover { color: var(--ink); }
@media (max-width: 760px) {
  .top-inner { padding: 8px 16px; gap: 6px 10px; flex-wrap: wrap; }
  .brand-sub { display: none; }
  .brand-name { font-size: 20px; }
  .brand svg { width: 26px; height: 26px; }
  .tabs { order: 3; flex-basis: 100%; overflow-x: auto; margin: 0 -6px; }
  .tabs a { padding: 6px 10px; font-size: 14px; }
  .controls { margin-left: auto; gap: 6px; }
  .seg button { padding: 5px 7px; font-size: 12.5px; }
  .icon-btn { width: 30px; height: 30px; font-size: 13px; }
  .colorby { flex-wrap: nowrap; overflow-x: auto; padding-bottom: 4px; margin-right: -16px; padding-right: 16px; }
  .pt-tools { gap: 10px; }
}

main { max-width: 1320px; margin: 0 auto; padding: 20px 20px 80px; outline: none; }
@media (max-width: 480px) { main { padding: 16px 16px 64px; } }

/* ---------- shared bits ---------- */
.eyebrow { margin: 0 0 6px; font: 500 12px/1.3 var(--font-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--accent); }
h1.page { margin: 0; font: 700 clamp(30px, 4.4vw, 44px)/1.04 var(--font-display); letter-spacing: -.01em; text-wrap: balance; }
.lede { margin: 8px 0 0; color: var(--muted); max-width: 70ch; }
.card { background: var(--surface); border: 1px solid var(--line); border-radius: var(--r); }
.chip { display: inline-flex; align-items: center; gap: 6px; padding: 5px 10px; border: 1px solid var(--line); background: var(--surface); border-radius: 999px; font-size: 13px; font-weight: 500; line-height: 1.2; white-space: nowrap; }
.chip:hover { border-color: var(--faint); }
.chip[aria-pressed="true"] { background: var(--ink); border-color: var(--ink); color: var(--bg); }
.btn { display: inline-flex; align-items: center; gap: 6px; padding: 8px 14px; border: 1px solid var(--line); background: var(--surface); border-radius: 8px; font-size: 14px; font-weight: 600; line-height: 1.2; }
.btn:hover { border-color: var(--faint); }
.btn.primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.btn.primary:hover { filter: brightness(1.06); }
.btn.small { padding: 5px 10px; font-size: 13px; }
.input { width: 100%; padding: 10px 12px; border: 1px solid var(--line-strong); background: var(--surface); border-radius: 8px; font-size: 16px; }
.input:focus { border-color: var(--accent); outline: none; box-shadow: 0 0 0 3px color-mix(in oklab, var(--accent) 25%, transparent); }
.input.formula { font-family: var(--font-mono); letter-spacing: .01em; }
.dot { width: 10px; height: 10px; border-radius: 3px; display: inline-block; background: var(--c); flex: none; }
.note { font-size: 13px; color: var(--muted); }
.lvl-tag { display: inline-block; font-size: 11px; font-weight: 600; line-height: 1; padding: 3px 6px; border-radius: 4px; background: var(--surface-3); color: var(--muted); vertical-align: 2px; }
.lvl-tag.core { background: var(--accent-soft); color: var(--accent); }
.lvl-tag.zk { background: color-mix(in oklab, var(--c-alkali) 16%, var(--surface)); color: var(--c-alkali); }
.lvl-tag.g10 { background: color-mix(in oklab, var(--c-halogen) 16%, var(--surface)); color: var(--c-halogen); }
.lvl-tag.hl { background: color-mix(in oklab, var(--c-noble, #8a6bd1) 18%, var(--surface)); color: var(--c-noble, #8a6bd1); }
.kind-tag { display: inline-block; font-size: 11px; font-weight: 600; line-height: 1; padding: 2px 6px; border-radius: 4px; border: 1px solid var(--line); color: var(--muted); white-space: nowrap; }
.rx-where { color: var(--muted); font-size: 13px; }
.eq .rad { font-weight: 700; margin-left: 1px; vertical-align: super; font-size: .8em; }
.rx-groups { margin-top: -4px; padding-top: 8px; border-top: 1px dashed var(--line); }
.rx-groups[hidden] { display: none; }
.rx-lib button.more { align-items: center; color: var(--accent); font-weight: 600; }
.lib-q { max-width: 240px; min-height: 34px; padding: 6px 10px; font-size: 14px; }
.lib-group.org h3::before { content: 'C · '; color: var(--faint); }

.cat-alkali { --c: var(--c-alkali); } .cat-alkaline { --c: var(--c-alkaline); } .cat-transition { --c: var(--c-transition); }
.cat-post { --c: var(--c-post); } .cat-metalloid { --c: var(--c-metalloid); } .cat-nonmetal { --c: var(--c-nonmetal); }
.cat-halogen { --c: var(--c-halogen); } .cat-noble { --c: var(--c-noble); } .cat-lanthanide { --c: var(--c-lanthanide); }
.cat-actinide { --c: var(--c-actinide); }

/* chemical equations */
.eq { display: inline-flex; flex-wrap: wrap; align-items: center; gap: 4px 8px; font-size: 17px; line-height: 1.3; }
.eq .sp { white-space: nowrap; }
.eq .coef { font-weight: 600; margin-right: 1px; }
.eq .plus { color: var(--muted); }
.arrow { display: inline-flex; flex-direction: column; align-items: center; min-width: 46px; padding: 0 2px; }
.arrow .cond { font-size: 11.5px; line-height: 1.1; color: var(--muted); white-space: nowrap; }
.arrow .ln { position: relative; display: block; width: 100%; min-width: 42px; height: 10px; margin: 1px 0; }
.arrow .ln.arr::before { content: ""; position: absolute; left: 0; right: 2px; top: 4px; height: 1.6px; background: currentColor; }
.arrow .ln.arr::after { content: ""; position: absolute; right: 0; top: 0.5px; border: 4.5px solid transparent; border-left: 8px solid currentColor; border-right: 0; }
.arrow .ln.eqs::before, .arrow .ln.eqs::after { content: ""; position: absolute; left: 0; right: 0; height: 1.5px; background: currentColor; }
.arrow .ln.eqs::before { top: 2px; } .arrow .ln.eqs::after { top: 6.5px; }
.arrow .ln.rev::before { content: ""; position: absolute; left: 0; right: 0; top: 2px; height: 1.4px; background: currentColor; clip-path: polygon(0 0, 100% 0, 100% 100%, 0 100%); }
.arrow .ln.rev::after { content: ""; position: absolute; left: 0; right: 0; top: 6.5px; height: 1.4px; background: currentColor; }
.arrow .ln.rev { background: linear-gradient(to bottom left, transparent 45%, currentColor 45% 55%, transparent 55%) right 0 top -2px / 7px 5px no-repeat, linear-gradient(to top right, transparent 45%, currentColor 45% 55%, transparent 55%) left 0 bottom -2px / 7px 5px no-repeat; }
.gas, .ppt { color: var(--accent); font-weight: 600; margin-left: 1px; }

/* ---------- periodic table view ---------- */
.pt-head { display: flex; flex-wrap: wrap; gap: 14px 24px; align-items: end; justify-content: space-between; margin-bottom: 14px; }
.pt-tools { display: flex; flex-wrap: wrap; gap: 10px 16px; align-items: center; margin-bottom: 12px; }
.search { position: relative; width: min(300px, 100%); }
.search .input { padding-left: 34px; font-size: 15px; }
.search svg { position: absolute; left: 11px; top: 50%; width: 16px; height: 16px; transform: translateY(-50%); stroke: var(--faint); fill: none; stroke-width: 2; }
.search-results { position: absolute; left: 0; right: 0; top: calc(100% + 4px); z-index: 12; background: var(--surface); border: 1px solid var(--line); border-radius: 8px; box-shadow: var(--shadow); padding: 4px; max-height: 320px; overflow: auto; }
.search-results button { display: flex; width: 100%; gap: 10px; align-items: baseline; text-align: left; border: 0; background: none; padding: 7px 9px; border-radius: 6px; }
.search-results button:hover, .search-results button.on { background: var(--surface-2); }
.search-results b { font: 600 15px/1 var(--font-display); min-width: 28px; }
.colorby { display: flex; flex-wrap: wrap; gap: 6px; align-items: center; }
.colorby .label { font: 500 11px/1 var(--font-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--muted); margin-right: 2px; }
.switch { display: inline-flex; align-items: center; gap: 8px; font-size: 13.5px; font-weight: 500; cursor: pointer; user-select: none; }
.switch input { appearance: none; -webkit-appearance: none; width: 34px; height: 20px; border-radius: 99px; background: var(--line-strong); position: relative; margin: 0; cursor: pointer; transition: background .15s; }
.switch input::after { content: ""; position: absolute; top: 2px; left: 2px; width: 16px; height: 16px; border-radius: 50%; background: var(--surface); box-shadow: 0 1px 2px rgba(0,0,0,.2); transition: transform .15s; }
.switch input:checked { background: var(--accent); }
.switch input:checked::after { transform: translateX(14px); }

.pt-scroll { overflow-x: auto; overflow-y: hidden; padding-bottom: 6px; margin: 0 -4px; -webkit-overflow-scrolling: touch; }
.pt { position: relative; display: grid; grid-template-columns: repeat(18, minmax(50px, 1fr)); grid-auto-rows: auto; gap: 4px; padding: 4px; min-width: 940px; }
.pt .gap-row { grid-column: 1 / -1; height: 10px; }
.tile {
  position: relative; aspect-ratio: 1 / 1.08; min-width: 0; border: 1px solid color-mix(in oklab, var(--c, var(--faint)) 45%, var(--line));
  background: var(--tile-bg, color-mix(in oklab, var(--c, var(--faint)) var(--tile-mix), var(--surface))); color: var(--tile-ink, var(--ink));
  border-radius: 6px; padding: 4px 5px 3px; text-align: left; display: flex; flex-direction: column; justify-content: space-between;
  transition: transform .12s ease, box-shadow .12s ease, opacity .2s ease; overflow: hidden;
}
.tile:hover, .tile:focus-visible { transform: translateY(-2px) scale(1.06); box-shadow: var(--shadow); z-index: 3; }
.tile .z { font: 500 10px/1 var(--font-mono); opacity: .8; }
.tile .sym { font: 700 clamp(17px, 1.75vw, 24px)/1 var(--font-display); letter-spacing: -.01em; }
.tile .nm { font-size: clamp(8.5px, .72vw, 10.5px); line-height: 1.1; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.tile .val { font: 400 clamp(8px, .66vw, 10px)/1 var(--font-mono); opacity: .78; white-space: nowrap; overflow: hidden; }
:lang(zh-CN) .tile .nm { font-size: clamp(10px, .9vw, 13px); font-weight: 500; }
.tile.dim { opacity: .28; }
.tile.dim:hover { opacity: 1; }
.tile.hit { box-shadow: 0 0 0 2px var(--accent); z-index: 2; }
.tile.nodata { background: repeating-linear-gradient(135deg, var(--surface-2) 0 5px, var(--surface) 5px 10px); border-color: var(--line); }
.tile.placeholder { background: transparent; border-style: dashed; color: var(--muted); justify-content: center; align-items: center; text-align: center; font-size: 10px; line-height: 1.25; cursor: default; }
.tile.placeholder:hover { transform: none; box-shadow: none; }
.tile.selected { box-shadow: 0 0 0 2.5px var(--ink); z-index: 4; }
.grp-num { font: 500 10.5px/1 var(--font-mono); color: var(--faint); text-align: center; align-self: end; padding-bottom: 2px; }
.per-num { font: 500 10.5px/1 var(--font-mono); color: var(--faint); display: grid; place-items: center; }

/* hover card inside the table gap */
.peek { grid-column: 3 / 13; grid-row: 2 / 5; align-self: stretch; display: grid; grid-template-columns: auto 1fr; gap: 6px 18px; align-items: center; padding: 6px 14px; pointer-events: none; }
.peek-tile { width: 108px; aspect-ratio: 1 / 1.08; border-radius: 8px; border: 1px solid color-mix(in oklab, var(--c) 50%, var(--line)); background: color-mix(in oklab, var(--c) var(--tile-mix), var(--surface)); padding: 8px 9px; display: flex; flex-direction: column; justify-content: space-between; }
.peek-tile .z { font: 500 12px/1 var(--font-mono); }
.peek-tile .sym { font: 700 44px/1 var(--font-display); }
.peek-tile .m { font: 400 11px/1 var(--font-mono); opacity: .8; }
.peek-info { min-width: 0; }
.peek-name { font: 700 24px/1.1 var(--font-display); }
.peek-name span { font-family: var(--font-body); font-weight: 500; font-size: 18px; color: var(--muted); margin-left: 6px; }
.peek-cat { font-size: 13px; color: var(--muted); display: flex; align-items: center; gap: 6px; margin-top: 3px; }
.peek-rows { margin-top: 8px; display: grid; grid-template-columns: repeat(3, auto); gap: 2px 18px; justify-content: start; font-size: 12.5px; }
.peek-rows dt { color: var(--muted); font-size: 11px; }
.peek-rows dd { margin: 0; font-family: var(--font-mono); }
.peek-hint { font-size: 12.5px; color: var(--faint); }

.legend { display: flex; flex-wrap: wrap; gap: 8px 16px; margin-top: 12px; font-size: 13px; color: var(--muted); align-items: center; }
.legend span { display: inline-flex; align-items: center; gap: 6px; }
.legend button { border: 0; background: none; padding: 2px 4px; border-radius: 5px; display: inline-flex; align-items: center; gap: 6px; color: var(--muted); font-size: 13px; }
.legend button:hover { color: var(--ink); background: var(--surface); }
.legend button[aria-pressed="true"] { color: var(--ink); background: var(--surface); box-shadow: inset 0 0 0 1px var(--line); }
.ramp { display: inline-block; width: 160px; height: 10px; border-radius: 3px; background: linear-gradient(90deg, var(--ramp-0), var(--ramp-1), var(--ramp-2), var(--ramp-3), var(--ramp-4)); border: 1px solid var(--line); }
.pt-foot { margin-top: 18px; display: grid; gap: 14px; grid-template-columns: repeat(auto-fit, minmax(260px, 1fr)); }
.pt-foot .card { padding: 14px 16px; }
.pt-foot h3 { margin: 0 0 4px; font: 700 18px/1.2 var(--font-display); }
.pt-foot p { margin: 0; color: var(--muted); font-size: 14px; }
@media (max-width: 1000px) { .peek { display: none; } }

/* ---------- element panel ---------- */
.scrim { position: fixed; inset: 0; background: var(--scrim); z-index: 40; opacity: 0; transition: opacity .2s ease; }
.scrim.open { opacity: 1; }
.panel {
  position: fixed; top: 0; right: 0; bottom: 0; z-index: 41; width: min(620px, 100%); background: var(--bg); border-left: 1px solid var(--line);
  box-shadow: var(--shadow); display: flex; flex-direction: column; transform: translateX(24px); opacity: 0; transition: transform .22s ease, opacity .22s ease;
  padding-top: env(safe-area-inset-top, 0px);
}
.panel.open { transform: none; opacity: 1; }
.panel-bar { display: flex; align-items: center; gap: 8px; padding: 10px 14px; border-bottom: 1px solid var(--line); background: var(--surface); }
.panel-bar .title { flex: 1; min-width: 0; font: 600 14px/1.2 var(--font-body); color: var(--muted); white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.panel-body { flex: 1; overflow-y: auto; overscroll-behavior: contain; padding: 18px 20px 48px; display: flex; flex-direction: column; gap: 18px; }
.panel-body > * { flex-shrink: 0; }
@media (max-width: 480px) { .panel-body { padding: 14px 16px 48px; } }
.el-head { display: grid; grid-template-columns: 96px 1fr; gap: 16px; align-items: center; }
.el-tile { aspect-ratio: 1 / 1.08; border-radius: 9px; border: 1px solid color-mix(in oklab, var(--c) 50%, var(--line)); background: color-mix(in oklab, var(--c) var(--tile-mix), var(--surface)); padding: 7px 8px; display: flex; flex-direction: column; justify-content: space-between; }
.el-tile .z { font: 500 12px/1 var(--font-mono); }
.el-tile .sym { font: 700 42px/1 var(--font-display); }
.el-tile .m { font: 400 10.5px/1 var(--font-mono); opacity: .8; }
.el-name { margin: 0; font: 700 32px/1.05 var(--font-display); }
.el-name .zh { font-family: var(--font-body); font-weight: 500; font-size: 22px; margin-left: 8px; }
.el-name .py { font-family: var(--font-body); font-weight: 400; font-size: 14px; color: var(--muted); margin-left: 4px; }
.el-meta { display: flex; flex-wrap: wrap; gap: 6px 14px; margin-top: 6px; font-size: 13.5px; color: var(--muted); align-items: center; }
.el-meta .cat { display: inline-flex; align-items: center; gap: 6px; color: var(--ink); font-weight: 500; }
.stage { position: relative; background: var(--surface); border: 1px solid var(--line); border-radius: var(--r); overflow: hidden; }
.stage-tabs { display: flex; gap: 2px; padding: 6px; border-bottom: 1px solid var(--line); overflow-x: auto; }
.stage-tabs button { border: 0; background: none; padding: 6px 10px; border-radius: 6px; font-size: 13px; font-weight: 600; color: var(--muted); white-space: nowrap; }
.stage-tabs button[aria-selected="true"] { background: var(--surface-3); color: var(--ink); }
.stage-view { position: relative; height: 340px; display: grid; place-items: center; }
.stage-view.tall { height: auto; min-height: 300px; padding: 16px; place-items: stretch; }
@media (max-width: 480px) { .stage-view { height: 300px; } }
.stage-view canvas { display: block; width: 100% !important; height: 100% !important; touch-action: none; }
.stage-view > svg { width: 100%; height: 100%; }
.stage-cap { display: flex; flex-wrap: wrap; gap: 8px 14px; align-items: center; justify-content: space-between; padding: 8px 12px; border-top: 1px solid var(--line); font-size: 13px; color: var(--muted); }
.stage-cap b { color: var(--ink); font-weight: 600; }
.ion-row { display: flex; gap: 6px; flex-wrap: wrap; align-items: center; }
.ion-row .label { font: 500 11px/1 var(--font-mono); letter-spacing: .06em; text-transform: uppercase; color: var(--muted); margin-right: 2px; }
.stage-overlay { position: absolute; left: 10px; top: 10px; font: 500 11.5px/1.35 var(--font-mono); color: var(--muted); pointer-events: none; }
.stage-overlay .k { display: inline-flex; align-items: center; gap: 5px; margin-right: 10px; }
.stage-overlay i { width: 9px; height: 9px; border-radius: 50%; display: inline-block; }
.stage-hint { position: absolute; right: 10px; bottom: 8px; font-size: 11.5px; color: var(--faint); pointer-events: none; }
.stage-msg { color: var(--muted); font-size: 14px; text-align: center; padding: 20px; max-width: 40ch; }
.stage-btns { position: absolute; right: 8px; top: 8px; display: flex; gap: 4px; }
.stage-btns button { border: 1px solid var(--line); background: color-mix(in oklab, var(--surface) 85%, transparent); border-radius: 6px; font-size: 12px; padding: 4px 8px; font-weight: 600; color: var(--muted); }
.stage-btns button[aria-pressed="true"] { color: var(--ink); border-color: var(--faint); }

.facts { display: grid; grid-template-columns: repeat(auto-fill, minmax(150px, 1fr)); gap: 1px; background: var(--line); border: 1px solid var(--line); border-radius: var(--r); overflow: hidden; }
.fact { background: var(--surface); padding: 9px 12px; min-width: 0; }
.fact dt { font-size: 11.5px; color: var(--muted); line-height: 1.3; }
.fact dd { margin: 2px 0 0; font: 500 15px/1.3 var(--font-mono); font-variant-numeric: tabular-nums; overflow-wrap: anywhere; }
.fact dd.txt { font-family: var(--font-body); font-size: 14.5px; }
.fact dd small { font-size: 11.5px; color: var(--muted); font-family: var(--font-body); }
.val-list { display: flex; gap: 4px; flex-wrap: wrap; }
.val-list span { font: 600 13px/1 var(--font-mono); padding: 3px 6px; border-radius: 4px; background: var(--accent-soft); color: var(--accent); }
.val-list span.other { background: var(--surface-3); color: var(--muted); font-weight: 500; }
.flame { display: inline-block; width: 10px; height: 14px; border-radius: 50% 50% 45% 45% / 70% 70% 30% 30%; background: radial-gradient(circle at 50% 70%, #fff8 0 20%, var(--fc) 60%); vertical-align: -2px; margin-right: 5px; }

.sec h2 { margin: 0 0 8px; font: 700 21px/1.2 var(--font-display); }
.sec h3 { margin: 14px 0 4px; font: 600 14px/1.3 var(--font-body); color: var(--muted); }
.sec p { margin: 0 0 8px; }
.sec ul { margin: 0; padding-left: 18px; }
.sec li { margin: 3px 0; }
.about-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 4px 20px; }
@media (max-width: 520px) { .about-grid { grid-template-columns: 1fr; } }
.callout { border-radius: 8px; padding: 10px 12px; background: var(--bad-soft); color: var(--ink); font-size: 14px; display: flex; gap: 8px; }
.callout::before { content: "⚠"; color: var(--bad); font-weight: 700; }
.callout.fact-c { background: var(--accent-soft); }
.callout.fact-c::before { content: "★"; color: var(--accent); }
.rx-list { display: flex; flex-direction: column; gap: 8px; }
.rx { background: var(--surface); border: 1px solid var(--line); border-radius: 8px; padding: 10px 12px; }
.rx .desc { font-size: 13.5px; color: var(--muted); margin-top: 4px; }
.rx .eq { font-size: 16px; }
.exam-list { list-style: none; padding: 0 !important; display: flex; flex-direction: column; gap: 6px; }
.exam-list li { display: grid; grid-template-columns: auto 1fr; gap: 10px; align-items: baseline; background: var(--surface); border: 1px solid var(--line); border-radius: 8px; padding: 8px 12px; margin: 0 !important; font-size: 14px; }
.cfg { font: 500 15px/1.6 var(--font-mono); overflow-wrap: anywhere; }
.cfg .core { color: var(--muted); }
.src { font-size: 12px; color: var(--faint); }

/* ---------- SVG diagrams ---------- */
.dg text { font-family: var(--font-mono); fill: var(--ink); }
.dg .ring { fill: none; stroke: var(--line-strong); stroke-width: 1.2; }
.dg .nuc { fill: color-mix(in oklab, var(--proton) 22%, var(--surface)); stroke: var(--proton); stroke-width: 1.5; }
.dg .e { fill: var(--electron); }
.dg .e-gain { stroke: var(--electron-gain); stroke-width: 2.2; stroke-linecap: round; }
.dg .spin { transform-box: view-box; transform-origin: center; animation: spin var(--dur, 30s) linear infinite; }
.dg .arc { fill: none; stroke: var(--ink); stroke-width: 1.6; }
.dg .muted { fill: var(--muted); }
@keyframes spin { to { transform: rotate(360deg); } }
@media (prefers-reduced-motion: reduce) { .dg .spin { animation: none; } }
.obox { display: flex; flex-wrap: wrap; gap: 14px 10px; align-items: flex-end; }
.obox .sub { display: flex; flex-direction: column; align-items: center; gap: 4px; }
.obox .boxes { display: flex; }
.obox .b { width: 26px; height: 30px; border: 1.5px solid var(--ink); margin-left: -1.5px; display: grid; place-items: center; font: 600 15px/1 var(--font-mono); letter-spacing: -2px; color: var(--electron); }
.obox .b:first-child { margin-left: 0; }
.obox .b.gain { color: var(--electron-gain); }
.obox .lbl { font: 500 12.5px/1 var(--font-mono); color: var(--muted); }
.obox .sub.valence .lbl { color: var(--accent); font-weight: 600; }

/* ---------- substances + reactions ---------- */
.layout-2 { display: grid; grid-template-columns: minmax(0, 1.1fr) minmax(0, 1fr); gap: 20px; align-items: start; }
@media (max-width: 960px) { .layout-2 { grid-template-columns: 1fr; } }
.formbar { display: flex; gap: 8px; align-items: stretch; flex-wrap: wrap; }
.formbar .input { flex: 1 1 260px; }
.examples { display: flex; flex-wrap: wrap; gap: 6px; margin-top: 10px; align-items: center; }
.examples .label { font-size: 12.5px; color: var(--muted); margin-right: 4px; }
.examples button { font-family: var(--font-body); }
.err { color: var(--bad); font-size: 14px; margin-top: 8px; }
.okmsg { color: var(--good); font-size: 14px; margin-top: 8px; }
.sub-head { display: flex; gap: 16px; align-items: baseline; flex-wrap: wrap; }
.sub-formula { font: 700 40px/1.05 var(--font-display); letter-spacing: .01em; }
.sub-names { font-size: 18px; font-weight: 600; }
.sub-names .alt { font-weight: 400; color: var(--muted); font-size: 14px; }
.badges { display: flex; flex-wrap: wrap; gap: 6px; margin-top: 6px; }
.badge { font-size: 12.5px; font-weight: 600; padding: 3px 8px; border-radius: 5px; background: var(--surface-3); color: var(--muted); }
.badge.k-ionic { background: color-mix(in oklab, var(--c-alkali) 16%, var(--surface)); color: var(--c-alkali); }
.badge.k-molecule { background: color-mix(in oklab, var(--c-nonmetal) 16%, var(--surface)); color: var(--c-nonmetal); }
.badge.k-metal { background: color-mix(in oklab, var(--c-transition) 18%, var(--surface)); color: color-mix(in oklab, var(--c-transition) 70%, var(--ink)); }
.badge.k-giant { background: color-mix(in oklab, var(--c-metalloid) 16%, var(--surface)); color: var(--c-metalloid); }
.badge.k-ion { background: color-mix(in oklab, var(--c-halogen) 16%, var(--surface)); color: var(--c-halogen); }
.valence-f { display: inline-flex; align-items: flex-end; gap: 1px; font: 600 30px/1 var(--font-display); padding-top: 22px; }
.valence-f .atom { position: relative; display: inline-flex; align-items: flex-end; }
.valence-f .ox { position: absolute; left: 50%; bottom: 100%; transform: translateX(-50%); font: 600 13px/1 var(--font-mono); color: var(--accent); white-space: nowrap; padding-bottom: 4px; }
.valence-f .ox.neg { color: var(--c-alkali); }
.valence-f sub { font-size: .55em; }
.table { width: 100%; border-collapse: collapse; font-size: 14px; }
.table th, .table td { text-align: left; padding: 7px 10px; border-bottom: 1px solid var(--line); vertical-align: top; }
.table th { font-size: 12px; font-weight: 600; color: var(--muted); }
.table td.num, .table th.num { text-align: right; font-family: var(--font-mono); font-variant-numeric: tabular-nums; }
.table tr:last-child td { border-bottom: 0; }
.table .ok { color: var(--good); font-weight: 600; }
.table .bad { color: var(--bad); font-weight: 600; }
.lib { display: flex; flex-direction: column; gap: 14px; }
.lib-group h3 { margin: 0 0 6px; font: 600 12px/1 var(--font-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--muted); }
.lib-items { display: flex; flex-wrap: wrap; gap: 6px; }
.lib-items button { border: 1px solid var(--line); background: var(--surface); border-radius: 7px; padding: 5px 9px; font-size: 13.5px; display: inline-flex; gap: 6px; align-items: baseline; text-align: left; }
.lib-items button:hover { border-color: var(--faint); }
.lib-items button .f { font-weight: 600; }
.lib-items button .n { color: var(--muted); font-size: 12.5px; }
.panel-card { padding: 16px 18px; }
.panel-card > h2 { margin: 0 0 10px; font: 700 20px/1.2 var(--font-display); }
.stack { display: flex; flex-direction: column; gap: 16px; }
.kv { display: grid; grid-template-columns: max-content 1fr; gap: 6px 16px; margin: 0; font-size: 14px; }
.kv dt { color: var(--muted); }
.kv dd { margin: 0; }
.bar-row { display: grid; grid-template-columns: 38px 1fr 64px; gap: 10px; align-items: center; font-size: 13.5px; margin: 5px 0; }
.bar-row .bar { height: 12px; background: var(--surface-3); border-radius: 3px; overflow: hidden; }
.bar-row .bar span { display: block; height: 100%; background: var(--c, var(--accent)); border-radius: 3px; }
.steps { margin: 0; padding: 0; list-style: none; display: flex; flex-direction: column; gap: 6px; font-size: 15px; }
.steps li { display: grid; grid-template-columns: 34px 1fr; gap: 10px; align-items: baseline; }
.steps .k { font: 600 13px/1 var(--font-body); color: var(--accent); background: var(--accent-soft); border-radius: 5px; padding: 4px 0; text-align: center; }
.steps .frac { display: inline-flex; flex-direction: column; align-items: center; vertical-align: middle; margin: 0 4px; font-family: var(--font-mono); font-size: 14px; }
.steps .frac span:first-child { border-bottom: 1.2px solid currentColor; padding: 0 4px; }
.qty-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(190px, 1fr)); gap: 10px; }
.qty { border: 1px solid var(--line); border-radius: 8px; padding: 10px; background: var(--surface-2); }
.qty .f { font: 600 18px/1.2 var(--font-display); }
.qty .row { display: flex; gap: 6px; margin-top: 6px; }
.qty .row input { width: 100%; min-width: 0; padding: 6px 8px; border: 1px solid var(--line-strong); border-radius: 6px; background: var(--surface); font-family: var(--font-mono); font-size: 14px; }
.qty .row select { padding: 6px; border: 1px solid var(--line-strong); border-radius: 6px; background: var(--surface); font-size: 13px; }
.qty .out { margin-top: 6px; font: 400 13px/1.45 var(--font-mono); color: var(--muted); }
.qty .out b { color: var(--ink); font-weight: 600; }
.qty.limiting { border-color: var(--bad); }
.qty.given { border-color: var(--accent); background: var(--accent-soft); }
.tag-lim { font-size: 11px; font-weight: 700; color: var(--bad); margin-left: 6px; }
.particle-stage { position: relative; height: 320px; background: var(--surface-2); border-radius: 8px; overflow: hidden; }
.particle-stage canvas { display: block; width: 100%; height: 100%; }
.particle-ctl { display: flex; gap: 8px; align-items: center; margin-top: 10px; flex-wrap: wrap; }
.particle-ctl input[type=range] { flex: 1 1 160px; accent-color: var(--accent); }
.rx-lib-filter { display: flex; flex-wrap: wrap; gap: 6px; margin-bottom: 12px; }
.rx-lib { display: flex; flex-direction: column; gap: 8px; max-height: 720px; overflow-y: auto; padding-right: 4px; }
.rx-lib button { text-align: left; border: 1px solid var(--line); background: var(--surface); border-radius: 8px; padding: 9px 12px; display: flex; flex-direction: column; gap: 4px; }
.rx-lib button:hover { border-color: var(--faint); }
.rx-lib .nm { font-size: 13px; color: var(--muted); display: flex; flex-wrap: wrap; gap: 6px 8px; align-items: center; }
.redox-line { font-size: 14px; }
.shift { display: inline-flex; gap: 4px; align-items: center; font-family: var(--font-mono); }
.shift .up { color: var(--c-alkali); font-weight: 600; }
.shift .down { color: var(--c-nonmetal); font-weight: 600; }
.lewis text { font-family: var(--font-display); font-weight: 600; fill: var(--ink); }
.lewis .d { fill: var(--electron); }
.lewis .x { stroke: var(--electron-gain); stroke-width: 1.6; stroke-linecap: round; }
.lewis .br { fill: none; stroke: var(--ink); stroke-width: 1.4; }
.lewis .chg { font-family: var(--font-mono); font-size: 13px; }
.lewis .bond { stroke: var(--ink); stroke-width: 1.8; }
.empty { color: var(--muted); padding: 24px; text-align: center; border: 1px dashed var(--line-strong); border-radius: var(--r); }
.foot { margin-top: 32px; font-size: 12.5px; color: var(--faint); }
.atom-label { font: 600 12px/1 var(--font-body); color: var(--ink); padding: 1px 4px; border-radius: 4px; background: color-mix(in oklab, var(--surface) 70%, transparent); transform: translateY(-16px); pointer-events: none; white-space: nowrap; }
.stage-view .stage-btns { z-index: 2; }
.stage-view .stage-overlay { z-index: 2; }
.examples .chip { font-size: 13px; }
.valence-f .atom { margin: 0 3px; }
.stage-view .lewis { display: block; margin: auto; }
.lib-items button .f sub { font-size: .7em; }
.particle-stage .stage-msg { position: absolute; inset: 0; display: grid; place-items: center; }
a.rx { display: block; color: inherit; text-decoration: none; }
a.rx:hover { border-color: var(--faint); }
#sub-rx:empty { display: none; }
.lib-items .chip { gap: 6px; text-decoration: none; color: var(--ink); }
.site-foot { max-width: 1320px; margin: 0 auto; padding: 0 20px 40px; font-size: 12.5px; color: var(--faint); }
.site-foot a { color: var(--muted); }
@media (max-width: 480px) { .site-foot { padding: 0 16px 32px; } }

/* ================= physics ================= */
.tabs { align-items: center; }
.seg.subject { margin-right: 8px; }
.seg.subject button { font-size: 13.5px; padding: 5px 11px; }
html:not([data-subject="phys"]) .tabs [data-subj="phys"], html[data-subject="phys"] .tabs [data-subj="chem"] { display: none; }
.frac { display: inline-flex; flex-direction: column; align-items: center; vertical-align: middle; margin: 0 3px; line-height: 1.25; }
.frac > span:first-child { border-bottom: 1.3px solid currentColor; padding: 0 4px 1px; }
.frac > span:last-child { padding: 1px 4px 0; }
.rad { border-top: 1.3px solid currentColor; padding: 0 2px; margin-left: 1px; }

/* solver */
.solver { display: flex; flex-direction: column; gap: 12px; }
.sv-laws { display: flex; flex-wrap: wrap; gap: 6px; }
.sv-formula { font: 600 24px/1.4 var(--font-body); display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.sv-formula sub { font-size: .6em; }
.sv-note { margin: -4px 0 0; }
.sv-rows { display: flex; flex-direction: column; gap: 6px; }
.sv-row { display: grid; grid-template-columns: 56px 64px minmax(0, 1fr) 150px 110px; gap: 6px 10px; align-items: center; padding: 6px 8px; border: 1px solid var(--line); border-radius: 8px; background: var(--surface-2); }
.sv-row.unknown { border-color: var(--accent); background: var(--accent-soft); }
.sv-find { border: 1px solid var(--line-strong); background: var(--surface); border-radius: 6px; padding: 4px 0; font-size: 12px; font-weight: 600; color: var(--muted); }
.sv-find[aria-pressed="true"] { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); font-size: 15px; }
.sv-sym { font: 600 18px/1 var(--font-body); }
.sv-name { font-size: 13.5px; color: var(--muted); min-width: 0; }
.sv-in { width: 100%; padding: 7px 9px; border: 1px solid var(--line-strong); border-radius: 6px; background: var(--surface); font: 500 15px var(--font-mono); }
.sv-in:disabled { background: transparent; border-style: dashed; }
.sv-in:focus { outline: none; border-color: var(--accent); box-shadow: 0 0 0 3px color-mix(in oklab, var(--accent) 25%, transparent); }
.sv-unit { padding: 7px 6px; border: 1px solid var(--line-strong); border-radius: 6px; background: var(--surface); font-size: 13.5px; }
.sv-unit-fixed { font-size: 13.5px; color: var(--muted); }
.sv-presets { grid-column: 3 / -1; display: flex; flex-wrap: wrap; gap: 4px; }
.mini { border: 1px solid var(--line); background: var(--surface); border-radius: 5px; padding: 2px 7px; font-size: 12px; color: var(--muted); }
.mini:hover { color: var(--ink); border-color: var(--faint); }
.sv-out .steps { font-size: 16px; }
.sv-out .steps li { grid-template-columns: 110px 1fr; }
.sv-out .steps .k { font-size: 12px; padding: 5px 4px; }
.sv-val { white-space: nowrap; }
.solver.compact .sv-formula { font-size: 21px; }
@media (max-width: 640px) {
  .sv-row { grid-template-columns: 48px 44px minmax(0, 1fr); }
  .sv-name { grid-column: 3; }
  .sv-in { grid-column: 1 / 3; }
  .sv-unit, .sv-unit-fixed { grid-column: 3; }
  .sv-presets { grid-column: 1 / -1; }
  .sv-out .steps li { grid-template-columns: 1fr; gap: 4px; }
  .sv-out .steps .k { justify-self: start; padding: 4px 8px; }
}

/* physics pages */
.ph-tools { display: flex; flex-wrap: wrap; gap: 10px; align-items: center; margin: 16px 0 6px; }
.ph-area { margin-top: 22px; }
.ph-area-head { display: flex; align-items: baseline; gap: 12px; flex-wrap: wrap; margin-bottom: 10px; }
.ph-area-head h2 { margin: 0; font: 700 26px/1.2 var(--font-display); }
.ph-list { list-style: none; margin: 0; padding: 0; background: var(--surface); border: 1px solid var(--line); border-radius: var(--r); overflow: hidden; }
.ph-list > li + li { border-top: 1px solid var(--line); }
.ph-row { display: grid; grid-template-columns: 26px 40px minmax(0, 1fr) auto 16px; align-items: center; gap: 14px; padding: 12px 16px; color: var(--ink); text-decoration: none; transition: background .15s; }
.ph-row:hover { background: var(--surface-2); }
.ph-row:focus-visible { outline: 2px solid var(--accent); outline-offset: -2px; }
.ph-row-n { display: inline-grid; place-items: center; width: 26px; height: 26px; border-radius: 50%; background: var(--surface-3); color: var(--muted); font-size: 12px; font-weight: 700; }
.ph-row:hover .ph-row-n { background: var(--accent); color: var(--accent-ink); }
.ph-row-main { display: flex; flex-direction: column; gap: 2px; min-width: 0; }
.ph-row-title { font: 700 17px/1.25 var(--font-display); }
.ph-row-line { font-size: 13.5px; color: var(--muted); }
.ph-row-f { font: 500 15px/1.4 var(--font-body); color: var(--accent); white-space: nowrap; text-align: right; }
.ph-row-f .sep { color: var(--faint); margin: 0 8px; }
.ph-row-mo { display: block; width: 40px; height: 40px; border-radius: 10px; background: var(--surface-2); }
.ph-row-mo .mo { width: 40px; height: 40px; }
.ph-row-go { width: 16px; height: 16px; fill: none; stroke: var(--faint); stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }
.ph-row:hover .ph-row-go { stroke: var(--accent); }
@media (max-width: 640px) {
  .ph-row { grid-template-columns: 40px minmax(0, 1fr) 14px; gap: 4px 12px; padding: 12px 14px; }
  .ph-row-n { display: none; }
  .ph-row-mo { grid-row: 1 / span 2; align-self: start; }
  .ph-row-f { grid-column: 2; grid-row: 2; white-space: normal; text-align: left; }
  .ph-row-go { grid-column: 3; grid-row: 1 / span 2; }
}
/* animated relationship SVGs (js/pviz/motion) */
.mo { display: block; width: 100%; height: auto; overflow: visible; --mo-hi: #d97706; }
@media (prefers-color-scheme: dark) { :root:not([data-theme="light"]) .mo { --mo-hi: #fbbf24; } }
:root[data-theme="dark"] .mo { --mo-hi: #fbbf24; }
.mo-paused .mo, .mo-paused .mo * { animation-play-state: paused !important; }
@media (prefers-reduced-motion: reduce) { .mo, .mo * { animation: none !important; } }
.ph-motion-head { display: flex; align-items: center; justify-content: space-between; gap: 12px; margin-bottom: 8px; }
.ph-motion-head h2 { margin: 0; font: 700 21px/1.2 var(--font-display); }
.ph-mo-host { max-width: 640px; margin: 0 auto; }
.ph-mo-cap { margin: 10px 0 0; }
.crumbs { font-size: 13px; color: var(--muted); margin-bottom: 8px; }
.crumbs a { color: var(--muted); }
.ph-title { display: flex; align-items: center; gap: 12px; flex-wrap: wrap; }
.ph-layout { display: flex; flex-direction: column; gap: 16px; margin-top: 18px; }
/* topic tabs: every topic of the current area, current one highlighted; scrolls sideways on phones */
.ph-topics { display: flex; gap: 6px; overflow-x: auto; margin: 2px 0 14px; padding: 2px 2px 8px; scrollbar-width: thin; -webkit-overflow-scrolling: touch; }
.ph-topic { display: inline-flex; align-items: center; gap: 7px; flex: none; padding: 6px 12px 6px 7px; border: 1px solid var(--line); border-radius: 999px; background: var(--surface); color: var(--ink); text-decoration: none; font-size: 13.5px; font-weight: 500; white-space: nowrap; }
.ph-topic .n { display: inline-grid; place-items: center; width: 20px; height: 20px; border-radius: 50%; background: var(--surface-3); color: var(--muted); font-size: 11px; font-weight: 700; }
.ph-topic:hover { border-color: var(--accent); }
.ph-topic[aria-current="page"] { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.ph-topic[aria-current="page"] .n { background: color-mix(in oklab, var(--accent-ink) 22%, transparent); color: var(--accent-ink); }
/* topic page: explanation on the left, calculator in a sticky column on the right */
.ph-wrap { container: phwrap / inline-size; }
.ph-split { display: grid; grid-template-columns: minmax(0, 1fr) clamp(300px, 40%, 440px); column-gap: 20px; row-gap: 0; margin-top: 18px; align-items: start; }
.ph-split > * { grid-column: 1; margin: 0 0 16px; }
.ph-split > .ph-side { grid-column: 2; grid-row: 1 / span 40; align-self: stretch; margin: 0; }
.ph-calc { position: sticky; top: calc(16px + var(--mc-bar-offset, 0px)); max-height: calc(100vh - 32px - var(--mc-bar-offset, 0px)); overflow-y: auto; overscroll-behavior: contain; display: flex; flex-direction: column; gap: 10px; border-top: 3px solid var(--accent); }
.ph-calc > h2 { margin: 0; font: 700 21px/1.2 var(--font-display); }
.ph-calc .sv-row { grid-template-columns: 48px 44px minmax(0, 1fr); }
.ph-calc .sv-name { grid-column: 3; }
.ph-calc .sv-in { grid-column: 1 / 3; }
.ph-calc .sv-unit, .ph-calc .sv-unit-fixed { grid-column: 3; justify-self: start; }
.ph-calc .sv-presets { grid-column: 1 / -1; }
.ph-calc .sv-out .steps li { grid-template-columns: 1fr; gap: 4px; }
.ph-calc .sv-out .steps .k { justify-self: start; padding: 4px 8px; }
.ph-calc .sv-laws .chip { white-space: normal; text-align: left; }
.ph-more { border-top: 1px solid var(--line); padding-top: 10px; }
.ph-more summary { cursor: pointer; font-size: 14px; font-weight: 600; color: var(--muted); }
.ph-more[open] summary { margin-bottom: 8px; }
.ph-more-list { max-height: 320px; overflow-y: auto; margin-top: 6px; }
.ph-laws button.ph-law { width: 100%; text-align: left; border: 1px solid transparent; background: none; border-radius: 8px; padding: 6px 10px; margin: 0 -10px; width: calc(100% + 20px); color: inherit; font: inherit; cursor: pointer; }
.ph-laws button.ph-law:hover:not([disabled]) { border-color: var(--line); background: var(--surface-2); }
.ph-laws button.ph-law.on { border-color: var(--accent); background: var(--accent-soft); }
.ph-laws button.ph-law[disabled] { cursor: default; }
.ph-law-go { margin-left: auto; color: var(--accent); font-weight: 700; opacity: 0; transition: opacity .15s; }
.ph-law:hover .ph-law-go, .ph-law.on .ph-law-go { opacity: 1; }
.ph-law-hint { margin: 8px 0 0; }
.ph-practice { display: flex; align-items: center; justify-content: space-between; gap: 12px; flex-wrap: wrap; }
.ph-practice h2 { margin: 0 0 4px; }
.ph-practice p { margin: 0; }
.ph-law-link.dim { opacity: .5; }
/* stack only when the page itself is narrow (phones, small windows), whatever the screen size */
@container phwrap (max-width: 600px) {
  .ph-split { display: flex; flex-direction: column; gap: 16px; }
  .ph-split > * { margin: 0; }
  .ph-calc { position: static; max-height: none; overflow: visible; }
}
.ph-sec > h2 { margin: 0 0 12px; font: 700 21px/1.2 var(--font-display); }
.ph-sec p { margin: 0 0 10px; max-width: 76ch; }
.ph-sec ul, .ph-sec ol { margin: 0; padding-left: 20px; }
.ph-sec li { margin: 4px 0; }
.ph-sub { margin: 16px 0 8px; font: 600 14px/1.3 var(--font-body); color: var(--muted); }
.ph-laws { display: flex; flex-direction: column; gap: 6px; }
.ph-law { display: flex; align-items: baseline; gap: 12px; flex-wrap: wrap; }
.ph-law.dim { opacity: .5; }
.ph-law-f { font: 600 19px/1.5 var(--font-body); }
.ph-law-f.small { font-size: 14px; font-weight: 500; color: var(--muted); }
.ph-ex { border: 1px solid var(--line); border-radius: 8px; padding: 10px 12px; margin-bottom: 8px; background: var(--surface-2); }
.ph-ex summary { cursor: pointer; font-weight: 500; }
.ph-ex .steps { margin-top: 10px; font-size: 15px; }
.ph-ans { font-weight: 600; margin: 8px 0 0 !important; color: var(--accent); }
.ph-mistakes li::marker { content: "✗  "; color: var(--bad); }
.ph-lab-link { margin: 12px 0 0; }
.ph-pager { display: flex; justify-content: space-between; gap: 10px; margin-top: 22px; flex-wrap: wrap; }
.ph-solve-layout { display: grid; grid-template-columns: 300px minmax(0, 1fr); gap: 16px; margin-top: 16px; align-items: start; }
@media (max-width: 860px) { .ph-solve-layout { grid-template-columns: 1fr; } }
.ph-lawlist { max-height: 70vh; overflow-y: auto; }
.ph-list-h { margin: 14px 0 4px; text-transform: uppercase; letter-spacing: .06em; font-size: 11.5px; }
.ph-law-link { display: flex; flex-direction: column; padding: 7px 9px; border-radius: 7px; text-decoration: none; color: var(--ink); font-size: 14px; }
.ph-law-link:hover { background: var(--surface-2); }
.ph-law-link.on { background: var(--accent-soft); }
.ph-model-part + .ph-model-part { margin-top: 18px; padding-top: 18px; border-top: 1px solid var(--line); }

/* shared model controls */
.cc-presets { display: flex; flex-wrap: wrap; gap: 6px; align-items: center; margin-bottom: 10px; }
.cc-presets .label { font: 500 11px/1 var(--font-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--muted); margin-right: 2px; }
.cc-top, .ramp-ctl { display: flex; flex-wrap: wrap; gap: 10px 18px; align-items: center; margin: 8px 0; }
.cc-field { display: inline-flex; align-items: center; gap: 8px; font-size: 13.5px; color: var(--muted); }
.cc-field input[type=number], .cc-field select { width: auto; max-width: 110px; padding: 5px 7px; border: 1px solid var(--line-strong); border-radius: 6px; background: var(--surface); color: var(--ink); font: 500 14px var(--font-mono); }
.cc-field select { font-family: var(--font-body); max-width: 190px; }
.cc-slide input[type=range] { width: 160px; accent-color: var(--accent); }
.cc-slide b { color: var(--ink); font: 600 13.5px var(--font-mono); min-width: 64px; }

/* circuit */
.cc-stage { overflow-x: auto; background: var(--surface-2); border-radius: 8px; padding: 6px; }
.schem { display: block; width: 100%; margin: 0 auto; }
.schem .wire { stroke: var(--ink); stroke-width: 2; fill: none; stroke-linecap: round; }
.schem .vm-wire { stroke: var(--muted); stroke-width: 1.5; }
.schem .flow { stroke: var(--c-alkaline); stroke-width: 3; stroke-dasharray: 3 12; stroke-linecap: round; fill: none; animation: flow 1s linear infinite; }
.schem .flow.rev { stroke: var(--electron); animation-direction: reverse; }
@keyframes flow { to { stroke-dashoffset: -30; } }
@media (prefers-reduced-motion: reduce) { .schem .flow { animation: none; } }
.schem .comp { fill: var(--surface); stroke: var(--ink); stroke-width: 2; }
.schem .comp-line { stroke: var(--ink); stroke-width: 2; fill: none; stroke-linecap: round; }
.schem .comp-ring { fill: var(--surface); stroke: var(--ink); stroke-width: 2; }
.schem .glow { fill: #ffd54a; filter: blur(4px); }
.schem .dotc { fill: var(--ink); }
.schem .meter { fill: var(--surface); }
.schem .meter-t { font: 700 13px var(--font-body); fill: var(--ink); }
.schem .lab { font: 500 11.5px var(--font-body); fill: var(--muted); }
.schem .lab .nm { fill: var(--ink); font-weight: 600; }
.schem .reading { font: 600 12px var(--font-mono); fill: var(--accent); }
.schem .sub-reading { font: 500 10.5px var(--font-mono); fill: var(--muted); }
.schem .hit { fill: transparent; stroke: none; }
.schem .comp-g { cursor: pointer; outline: none; }
.schem .comp-g:hover .hit, .schem .comp-g:focus-visible .hit { fill: color-mix(in oklab, var(--accent) 10%, transparent); stroke: var(--accent); stroke-dasharray: 4 3; }
.schem .comp-g.sel .hit { fill: color-mix(in oklab, var(--accent) 16%, transparent); stroke: var(--accent); }
.schem .par-box { fill: none; stroke: var(--line-strong); stroke-dasharray: 4 4; }
.schem .plate-long { stroke: var(--ink); stroke-width: 3; }
.schem .plate-short { stroke: var(--ink); stroke-width: 6; }
.cc-edit { margin: 10px 0; }
.cc-editor { display: flex; flex-wrap: wrap; gap: 10px 16px; align-items: center; padding: 10px 12px; border: 1px solid var(--accent); border-radius: 8px; background: var(--accent-soft); }
.cc-ops { display: flex; flex-wrap: wrap; gap: 6px; flex-basis: 100%; }
.cc-ops [aria-pressed="true"] { background: var(--ink); color: var(--bg); }
.cc-grid { display: grid; grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); gap: 16px; margin-top: 8px; }
@media (max-width: 760px) { .cc-grid { grid-template-columns: 1fr; } }
.cc-grid .table tr.on td { background: var(--accent-soft); }
.cc-grid .steps { font-size: 14px; }
.cc-grid .steps li { grid-template-columns: 22px 1fr; }

/* energy models */
.ramp-cv { width: 100%; height: 300px; display: block; background: var(--surface-2); border-radius: 8px; }
.sk-svg svg { width: 100%; max-width: 680px; display: block; }
.sk-lab { font: 500 12px var(--font-body); fill: var(--ink); }
.sk-lab.muted { fill: var(--muted); }
.sk-eff { font-size: 16px; }
.sk-table { max-width: 560px; }
.sk-in { width: 100%; padding: 4px 6px; border: 1px solid var(--line); border-radius: 5px; background: var(--surface); font-size: 13.5px; }
.sk-in.num { font-family: var(--font-mono); text-align: right; max-width: 110px; }
.chain-row { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; padding: 14px 4px; }
.chain-box { padding: 8px 12px; border-radius: 8px; border: 1px solid var(--line-strong); background: var(--surface); font-size: 14px; font-weight: 500; }
.chain-box.useful { border-color: var(--accent); background: var(--accent-soft); }
.chain-box.waste { border-color: color-mix(in oklab, var(--c-alkali) 50%, var(--line)); background: color-mix(in oklab, var(--c-alkali) 10%, var(--surface)); }
.chain-arrow { display: inline-flex; flex-direction: column; align-items: center; font-size: 11.5px; color: var(--muted); min-width: 44px; }
.chain-arrow::after { content: "⟶"; font-size: 20px; color: var(--ink); line-height: 1; }
.chain-arrow span { max-width: 130px; text-align: center; line-height: 1.2; }
.chain-outs { display: flex; flex-direction: column; gap: 6px; }
.calor-grid { display: grid; grid-template-columns: 200px minmax(0, 1fr); gap: 18px; align-items: start; }
@media (max-width: 560px) { .calor-grid { grid-template-columns: 1fr; } .calor-svg { max-width: 180px; } }
.calor-svg { width: 100%; }
.calor-svg .stand { fill: var(--line-strong); }
.calor-svg .tube { fill: color-mix(in oklab, var(--surface) 70%, transparent); stroke: var(--muted); stroke-width: 2; }
.calor-svg .water { fill: color-mix(in oklab, var(--c-nonmetal) 30%, transparent); }
.calor-svg .thermo { fill: var(--surface); stroke: var(--muted); }
.calor-svg .thermo-bulb, .calor-svg .thermo-fill { fill: var(--c-alkali); }
.calor-svg .needle { stroke: var(--muted); stroke-width: 2; }
.calor-svg .food { fill: #b8894a; }
.calor-svg .flame { fill: #ff9f1c; }
.calor-form { display: flex; flex-direction: column; gap: 8px; }
.calor-form .cc-field { justify-content: space-between; max-width: 340px; }
.calor .steps { font-size: 15px; margin: 12px 0; }
.wire-svg { width: 100%; max-width: 560px; display: block; }
.wire-svg .clip { fill: var(--c-alkali); opacity: .8; }
.wire-svg .lab { font: 500 12px var(--font-mono); fill: var(--muted); }
.wire-r { font-size: 17px; margin: 8px 0; }
.wire-notes { margin: 8px 0 0; padding-left: 20px; font-size: 14px; color: var(--muted); }
.iv-svg { width: 100%; max-width: 520px; display: block; background: var(--surface-2); border-radius: 8px; }
.iv-svg .axis { stroke: var(--ink); stroke-width: 1.4; }
.iv-svg .tick { font: 500 10.5px var(--font-mono); fill: var(--muted); }
.iv-svg .lab { font: 600 13px var(--font-body); fill: var(--ink); }
.iv-svg .cursor { stroke: var(--muted); stroke-dasharray: 4 4; }
.iv-read { list-style: none; padding: 0; margin: 6px 0; font: 500 13.5px var(--font-mono); }
.iv-read li { display: flex; align-items: center; gap: 6px; margin: 3px 0; }
a.btn { text-decoration: none; color: var(--ink); }
a.btn.primary { color: var(--accent-ink); }

/* ================= subject colours ================= */
/* fixed hues for each subject (used where both subjects appear together, e.g. the sidebar) */
:root { --chem-hue: #1e6b2e; --phys-hue: #0a0a0a; --bb-tile-a: #4b9a4d; --bb-tile-b: #1b5625; }
@media (prefers-color-scheme: dark) { :root:not([data-theme="light"]) { --chem-hue: #6ad16f; --phys-hue: #ffffff; --bb-tile-a: #3f9443; --bb-tile-b: #134a1c; } }
:root[data-theme="dark"] { --chem-hue: #6ad16f; --phys-hue: #ffffff; --bb-tile-a: #3f9443; --bb-tile-b: #134a1c; }

/* Chemistry: Breaking Bad — element-tile green, desert paper in light mode, near-black in dark mode */
:root[data-subject="chem"] {
  --bg: #f2f1e6; --grid: rgba(40, 90, 40, .06); --surface: #fffef8; --surface-2: #f7f5ea; --surface-3: #ecead9;
  --ink: #1a1f16; --muted: #59614f; --faint: #8b917f; --line: #dcd9c4; --line-strong: #c6c2a9;
  --accent: #1e6b2e; --accent-ink: #ffffff; --accent-soft: #deebd6; --good: #1e6b2e; --focus: #1e6b2e;
}
@media (prefers-color-scheme: dark) {
  :root[data-subject="chem"]:not([data-theme="light"]) {
    --bg: #0a0d09; --grid: rgba(120, 200, 120, .045); --surface: #111710; --surface-2: #151d14; --surface-3: #1b2519;
    --ink: #e6ede0; --muted: #9aaa92; --faint: #6d7c66; --line: #243022; --line-strong: #334331;
    --accent: #6ad16f; --accent-ink: #062109; --accent-soft: #15301a; --good: #6ad16f; --focus: #6ad16f;
  }
}
:root[data-subject="chem"][data-theme="dark"] {
  --bg: #0a0d09; --grid: rgba(120, 200, 120, .045); --surface: #111710; --surface-2: #151d14; --surface-3: #1b2519;
  --ink: #e6ede0; --muted: #9aaa92; --faint: #6d7c66; --line: #243022; --line-strong: #334331;
  --accent: #6ad16f; --accent-ink: #062109; --accent-soft: #15301a; --good: #6ad16f; --focus: #6ad16f;
}
/* Physics: black on white in light mode, white on black in dark mode */
:root[data-subject="phys"] {
  --bg: #ffffff; --grid: rgba(0, 0, 0, .04); --surface: #ffffff; --surface-2: #f6f6f6; --surface-3: #ececec;
  --ink: #0a0a0a; --muted: #555555; --faint: #8a8a8a; --line: #dedede; --line-strong: #c4c4c4;
  --accent: #0a0a0a; --accent-ink: #ffffff; --accent-soft: #ededed; --focus: #0a0a0a;
}
@media (prefers-color-scheme: dark) {
  :root[data-subject="phys"]:not([data-theme="light"]) {
    --bg: #000000; --grid: rgba(255, 255, 255, .04); --surface: #0e0e0e; --surface-2: #161616; --surface-3: #1f1f1f;
    --ink: #f4f4f4; --muted: #a6a6a6; --faint: #707070; --line: #262626; --line-strong: #3b3b3b;
    --accent: #ffffff; --accent-ink: #000000; --accent-soft: #262626; --focus: #ffffff;
  }
}
:root[data-subject="phys"][data-theme="dark"] {
  --bg: #000000; --grid: rgba(255, 255, 255, .04); --surface: #0e0e0e; --surface-2: #161616; --surface-3: #1f1f1f;
  --ink: #f4f4f4; --muted: #a6a6a6; --faint: #707070; --line: #262626; --line-strong: #3b3b3b;
  --accent: #ffffff; --accent-ink: #000000; --accent-soft: #262626; --focus: #ffffff;
}
:root[data-subject] .card { box-shadow: inset 0 2px 0 0 var(--accent); }
:root[data-subject="phys"] h1.page { padding-bottom: 10px; background: linear-gradient(var(--accent), var(--accent)) left bottom / 56px 4px no-repeat; }
:root[data-subject="chem"] h1.page { color: var(--ink); }
.bb-tile { position: relative; display: inline-flex; align-items: center; justify-content: center; min-width: 1.18em; height: 1.18em; padding: 0 .1em; margin-right: .05em;
  background: linear-gradient(155deg, var(--bb-tile-a), var(--bb-tile-b)); color: #fff; font-family: var(--font-display); vertical-align: -.1em;
  border: 2px solid rgba(255, 255, 255, .28); box-shadow: 0 0 0 1px var(--bb-tile-b); border-radius: 2px; }
.bb-tile sup { position: absolute; top: .16em; left: .16em; font: 500 .22em/1 var(--font-mono); }

/* ================= layout with sidebar ================= */
.top { display: none; }
.shell { display: grid; grid-template-columns: 252px minmax(0, 1fr); min-height: 100vh; }
html[data-sidebar="collapsed"] .shell { grid-template-columns: 66px minmax(0, 1fr); }
.main-col { min-width: 0; display: flex; flex-direction: column; }
main { width: 100%; padding: 24px 28px 80px; }
.site-foot { width: 100%; padding: 0 28px 40px; }
.topbar { display: none; }
.drawer-scrim { display: none; }
.sidebar { position: sticky; top: 0; height: 100vh; overflow-y: auto; overscroll-behavior: contain; background: var(--surface); border-right: 1px solid var(--line);
  display: flex; flex-direction: column; gap: 12px; padding: 14px 12px; z-index: 30; }
.ico { width: 18px; height: 18px; flex: none; fill: none; stroke: currentColor; stroke-width: 1.8; stroke-linecap: round; stroke-linejoin: round; }
.sb-head { display: flex; align-items: center; gap: 6px; }
.sb-brand { display: inline-flex; align-items: center; gap: 9px; text-decoration: none; color: var(--ink); font: 700 21px/1 var(--font-display); flex: 1; min-width: 0; padding: 4px; }
.brand-mark { width: 30px; height: 30px; flex: none; }
.brand-mark rect { fill: var(--accent); } .brand-mark circle { fill: var(--accent-ink); } .brand-mark g { stroke: var(--accent-ink); }
.sb-icon-btn { width: 34px; height: 34px; display: inline-grid; place-items: center; border: 1px solid transparent; background: none; border-radius: 8px; color: var(--muted); }
.sb-icon-btn:hover { background: var(--surface-2); color: var(--ink); }
.sb-close { display: none; }
.sb-search { display: flex; align-items: center; gap: 8px; width: 100%; padding: 8px 10px; border: 1px solid var(--line-strong); border-radius: 9px; background: var(--surface-2); color: var(--muted); font-size: 14px; }
.sb-search:hover { border-color: var(--accent); color: var(--ink); }
.sb-search kbd { margin-left: auto; }
kbd { font: 500 11px/1.4 var(--font-mono); border: 1px solid var(--line-strong); border-bottom-width: 2px; border-radius: 5px; padding: 0 5px; background: var(--surface); color: var(--muted); }
.sb-settings { display: flex; flex-direction: column; gap: 8px; padding: 10px; border: 1px solid var(--line); border-radius: 10px; background: var(--surface-2); }
.sb-grade { width: 100%; }
.sb-grade button { flex: 1; }
.sb-row { display: flex; gap: 6px; }
.sb-pill { flex: 1; display: inline-flex; align-items: center; justify-content: center; gap: 6px; height: 32px; border: 1px solid var(--line); background: var(--surface); border-radius: 7px; font-size: 13px; font-weight: 600; color: var(--muted); }
.sb-pill:hover { color: var(--ink); border-color: var(--line-strong); }
.sb-nav { display: flex; flex-direction: column; gap: 14px; }
.sb-group-t { display: flex; align-items: center; gap: 8px; margin: 0 10px 4px; font: 500 11px/1 var(--font-mono); letter-spacing: .1em; text-transform: uppercase; color: var(--muted); }
.sb-dot { width: 9px; height: 9px; border-radius: 2px; flex: none; }
.sb-group.chem .sb-dot { background: linear-gradient(155deg, var(--bb-tile-a), var(--bb-tile-b)); }
.sb-group.phys .sb-dot { background: var(--phys-hue); }
.sb-item { display: flex; align-items: center; gap: 10px; width: 100%; padding: 7px 10px; border: 0; background: none; border-radius: 8px; color: var(--muted); text-decoration: none; font-size: 14.5px; font-weight: 500; text-align: left; }
.sb-item:hover { background: var(--surface-2); color: var(--ink); }
.sb-item.on { background: var(--accent-soft); color: var(--accent); }
.sb-ext { margin-left: auto; font-size: 12px; opacity: .6; }
html[data-sidebar="collapsed"] .sb-ext { display: none; }
.sb-chev { margin-left: auto; display: inline-flex; transition: transform .15s; }
.sb-chev .ico { width: 15px; height: 15px; }
.sb-folder.open .sb-chev { transform: rotate(90deg); }
.sb-sub { display: none; margin: 2px 0 4px 21px; padding-left: 8px; border-left: 1.5px solid var(--line); }
.sb-folder.open .sb-sub { display: block; }
.sb-subitem { display: block; padding: 5px 9px; font-size: 13.5px; line-height: 1.3; color: var(--muted); text-decoration: none; border-radius: 6px; }
.sb-subitem:hover { color: var(--ink); background: var(--surface-2); }
.sb-subitem.on { color: var(--accent); background: var(--accent-soft); font-weight: 600; }
.sb-foot { margin-top: auto; padding-top: 10px; border-top: 1px solid var(--line); display: flex; flex-direction: column; gap: 2px; }
.sb-foot .sb-item { font-size: 13.5px; }
/* collapsed desktop sidebar: icons only */
html[data-sidebar="collapsed"] .sidebar { padding: 14px 8px; align-items: center; }
html[data-sidebar="collapsed"] .sb-label, html[data-sidebar="collapsed"] .sb-settings, html[data-sidebar="collapsed"] .sb-sub, html[data-sidebar="collapsed"] .sb-group-t { display: none; }
html[data-sidebar="collapsed"] .sb-head { flex-direction: column; }
html[data-sidebar="collapsed"] .sb-brand { flex: none; padding: 0; }
html[data-sidebar="collapsed"] .sb-item, html[data-sidebar="collapsed"] .sb-search { justify-content: center; padding: 9px 0; width: 44px; }
html[data-sidebar="collapsed"] .sb-group { display: flex; flex-direction: column; align-items: center; gap: 2px; padding-top: 8px; border-top: 1px solid var(--line); }
html[data-sidebar="collapsed"] .sb-collapse .ico { transform: scaleX(-1); }
/* phones and tablets: drawer + top bar */
@media (max-width: 1000px) {
  .shell, html[data-sidebar="collapsed"] .shell { grid-template-columns: minmax(0, 1fr); }
  .topbar { display: flex; position: sticky; top: 0; z-index: 25; align-items: center; gap: 8px; padding: 8px 12px; padding-top: calc(8px + env(safe-area-inset-top, 0px));
    background: color-mix(in oklab, var(--bg) 90%, transparent); backdrop-filter: blur(10px); -webkit-backdrop-filter: blur(10px); border-bottom: 1px solid var(--line); }
  .topbar .sb-brand { flex: none; font-size: 19px; }
  .topbar .brand-mark { width: 26px; height: 26px; }
  .tb-title { flex: 1; min-width: 0; font: 500 11.5px/1 var(--font-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--accent); }
  .sidebar, html[data-sidebar="collapsed"] .sidebar { position: fixed; left: 0; top: 0; bottom: 0; height: auto; width: min(300px, 86vw); padding: 14px 12px; align-items: stretch; transform: translateX(-104%); transition: transform .22s ease; box-shadow: var(--shadow); z-index: 45; padding-top: calc(14px + env(safe-area-inset-top, 0px)); }
  html[data-sidebar="collapsed"] .sb-label, html[data-sidebar="collapsed"] .sb-settings, html[data-sidebar="collapsed"] .sb-group-t { display: revert; }
  html[data-sidebar="collapsed"] .sb-settings { display: flex; }
  html[data-sidebar="collapsed"] .sb-folder.open .sb-sub { display: block; }
  html[data-sidebar="collapsed"] .sb-head { flex-direction: row; }
  html[data-sidebar="collapsed"] .sb-brand { flex: 1; }
  html[data-sidebar="collapsed"] .sb-item, html[data-sidebar="collapsed"] .sb-search { justify-content: flex-start; padding: 7px 10px; width: 100%; }
  html[data-sidebar="collapsed"] .sb-group { display: block; border: 0; padding: 0; }
  html[data-drawer="open"] .sidebar { transform: none; }
  html[data-drawer="open"] .drawer-scrim { display: block; position: fixed; inset: 0; background: var(--scrim); z-index: 44; }
  .sb-collapse { display: none; }
  .sb-close { display: inline-grid; }
  main { padding: 18px 16px 64px; }
  .site-foot { padding: 0 16px 32px; }
}
@media (prefers-reduced-motion: reduce) { .sidebar { transition: none !important; } }

/* ================= ⌘K search ================= */
.pal-scrim { position: fixed; inset: 0; z-index: 60; background: var(--scrim); display: flex; justify-content: center; align-items: flex-start; padding: 10vh 16px 16px; opacity: 0; transition: opacity .15s ease; }
.pal-scrim.open { opacity: 1; }
.pal { width: min(640px, 100%); max-height: 74vh; display: flex; flex-direction: column; background: var(--surface); border: 1px solid var(--line); border-radius: 14px; box-shadow: var(--shadow); overflow: hidden; }
.pal-in { display: flex; align-items: center; gap: 10px; padding: 12px 14px; border-bottom: 1px solid var(--line); color: var(--muted); }
.pal-in input { flex: 1; min-width: 0; border: 0; background: none; font-size: 17px; outline: none; color: var(--ink); }
.pal-res { overflow-y: auto; padding: 6px; }
.pal-res:empty { display: none; }
.pal-g { margin: 10px 10px 4px; font: 500 11px/1 var(--font-mono); letter-spacing: .1em; text-transform: uppercase; color: var(--muted); display: flex; align-items: center; gap: 6px; }
.pal-g.chem::before, .pal-g.phys::before { content: ""; width: 8px; height: 8px; border-radius: 2px; }
.pal-g.chem::before { background: var(--chem-hue); } .pal-g.phys::before { background: var(--phys-hue); }
.pal-item { display: flex; align-items: baseline; gap: 12px; padding: 8px 10px; border-radius: 8px; color: var(--ink); text-decoration: none; }
.pal-item.on, .pal-item:hover { background: var(--accent-soft); }
.pal-sym { min-width: 32px; font: 700 18px/1 var(--font-display); }
.pal-f { min-width: 70px; font: 600 16px/1.2 var(--font-body); }
.pal-rx { font-size: 14.5px; }
.pal-meta { margin-left: auto; color: var(--muted); font-size: 12.5px; text-align: right; }
.pal-empty { margin: 14px; color: var(--muted); font-size: 14px; }
.pal-foot { display: flex; gap: 16px; padding: 8px 14px; border-top: 1px solid var(--line); font-size: 12px; color: var(--muted); }
.pal-foot kbd { margin-right: 2px; }
@media (max-width: 600px) { .pal-foot { display: none; } .pal-scrim { padding-top: 8px; } .pal-meta { display: none; } }

/* ================= start screen ================= */
.welcome-scrim { align-items: center; padding: 16px; }
.welcome { width: min(540px, 100%); max-height: 92vh; overflow-y: auto; background: var(--surface); border: 1px solid var(--line); border-radius: 16px; padding: 24px 26px 22px; box-shadow: var(--shadow); }
.welcome h2 { margin: 0; font: 700 30px/1.1 var(--font-display); }
.wl-lede { margin: 6px 0 4px; color: var(--muted); }
.wl-q { margin: 16px 0 6px; font: 500 11px/1 var(--font-mono); letter-spacing: .1em; text-transform: uppercase; color: var(--muted); }
.wl-note { margin: 6px 0 0; font-size: 12.5px; color: var(--faint); }
.wl-grid { display: grid; gap: 8px; }
.wl-grid.three { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.wl-grid.two { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.wl-opt { display: flex; flex-direction: column; align-items: flex-start; gap: 3px; padding: 10px 12px; border: 1.5px solid var(--line-strong); border-radius: 10px; background: var(--surface-2); text-align: left; }
.wl-opt:hover { border-color: var(--accent); }
.wl-opt[aria-pressed="true"] { border-color: var(--accent); background: var(--accent-soft); box-shadow: inset 0 0 0 1px var(--accent); }
.wl-main { display: flex; align-items: center; gap: 8px; font: 600 16px/1.2 var(--font-body); }
.wl-sub { font-size: 12.5px; color: var(--muted); line-height: 1.3; }
.wl-chip { display: inline-grid; place-items: center; width: 26px; height: 26px; font: 700 13px/1 var(--font-display); color: #fff; border-radius: 3px; }
.wl-chip.chem { background: linear-gradient(155deg, var(--bb-tile-a), var(--bb-tile-b)); }
.wl-chip.phys { background: var(--phys-hue); color: var(--bg); }
.wl-actions { display: flex; justify-content: flex-end; gap: 8px; margin-top: 22px; }
@media (max-width: 480px) { .welcome { padding: 20px 18px; } .wl-grid.three { grid-template-columns: 1fr; } }

/* ================= calculator: answer first ================= */
.sv-tools { display: flex; flex-wrap: wrap; gap: 6px; }
.sv-answer { display: flex; flex-direction: column; gap: 3px; padding: 12px 14px; border-radius: 10px; background: var(--accent-soft); border: 1px solid color-mix(in oklab, var(--accent) 35%, transparent); }
.sv-ans-v { font: 500 22px/1.35 var(--font-body); }
.sv-ans-v b { font-weight: 700; }
.sv-work { margin-top: 10px; }
.sv-work summary { cursor: pointer; font-weight: 600; font-size: 14px; color: var(--accent); width: fit-content; }
.sv-work .steps { margin: 10px 0; }

/* ================= practice mode ================= */
.ph-ex-head { display: flex; justify-content: space-between; align-items: center; gap: 12px; flex-wrap: wrap; margin-bottom: 12px; }
.ph-ex-head h2 { margin: 0; font: 700 21px/1.2 var(--font-display); }
.px { border: 1px solid var(--line); border-radius: 10px; padding: 12px 14px; background: var(--surface-2); margin-bottom: 10px; }
.px-q { margin: 0 0 4px !important; font-weight: 500; }
.px-hint { margin: 0 0 6px !important; }
.px-try { display: flex; gap: 8px; align-items: center; max-width: 440px; }
.px-in { flex: 1; font-family: var(--font-mono); padding: 7px 10px; font-size: 15px; }
.px-fb { margin: 8px 0 0 !important; font-size: 14px; }
.px-fb:empty { display: none; }
.px-fb.ok { color: var(--good); font-weight: 600; }
.px-fb.bad { color: var(--bad); }
.px-steps { margin: 10px 0 4px; font-size: 15px; }
.px-steps:empty { display: none; }
.px-new { animation: pxin .25s ease; }
@keyframes pxin { from { opacity: 0; transform: translateY(-4px); } }
.px-btns { display: flex; flex-wrap: wrap; gap: 6px; margin-top: 8px; }
@media (prefers-reduced-motion: reduce) { .px-new { animation: none; } }

/* floating calculator button (opens muqing.ai/calc): a rectangle with the icon and the name */
.calc-fab { position: fixed; right: max(18px, env(safe-area-inset-right, 0px)); bottom: max(18px, env(safe-area-inset-bottom, 0px)); z-index: 35; display: inline-flex; align-items: center; gap: 9px; height: 48px; padding: 0 16px 0 12px; border-radius: 12px;
  background: var(--accent); color: var(--accent-ink); text-decoration: none; font-weight: 700; font-size: 15px; letter-spacing: .01em;
  box-shadow: 0 8px 22px rgba(0, 0, 0, .28), 0 0 0 1px color-mix(in oklab, var(--accent-ink) 14%, transparent) inset; transition: transform .15s ease, box-shadow .15s ease; }
.calc-fab svg { width: 24px; height: 24px; flex: none; fill: none; stroke: currentColor; stroke-width: 1.7; }
.calc-fab svg circle { fill: currentColor; stroke: none; }
.calc-fab-l { white-space: nowrap; }
.calc-fab:hover, .calc-fab:focus-visible { transform: translateY(-2px); box-shadow: 0 12px 26px rgba(0, 0, 0, .32); }
@media (max-width: 640px) { .calc-fab { width: 50px; height: 50px; padding: 0; justify-content: center; } .calc-fab-l { display: none; } }
/* dark mode: black and white */
@media (prefers-color-scheme: dark) { :root:not([data-theme="light"]) .calc-fab { background: #fff; color: #000; box-shadow: 0 8px 22px rgba(0, 0, 0, .55), 0 0 0 1px rgba(255, 255, 255, .25); } }
:root[data-theme="dark"] .calc-fab { background: #fff; color: #000; box-shadow: 0 8px 22px rgba(0, 0, 0, .55), 0 0 0 1px rgba(255, 255, 255, .25); }
html[data-drawer="open"] .calc-fab { display: none; }
@media print { .calc-fab { display: none; } }