:root {
    --bg: #F1F2EE;
    --surface: #FFFFFF;
    --sunk: #E7E9E3;
    --ink: #14171B;
    --ink-2: #48505A;
    --ink-3: #5F6670;
    --rule: #D7DAD2;
    --rule-2: #BCC1B8;
    --accent: #2340B8;
    --accent-soft: #E2E7FA;
    --add: #1C7444;
    --add-bg: #DCF0E3;
    --del: #AE3328;
    --del-bg: #F8E0DC;
    --hl: #865600;
    --hl-bg: #F6EACB;
    --code-bg: #F8F9F5;
    --tk-key: #2340B8;
    --tk-str: #0D6A6A;
    --tk-num: #965700;
    --tk-bool: #A2356E;
    --tk-com: #5F6670;
    --display: "Bricolage Grotesque", "Avenir Next", "Segoe UI", sans-serif;
    --body: "IBM Plex Sans", "Segoe UI", Helvetica, Arial, sans-serif;
    --mono: "IBM Plex Mono", ui-monospace, Menlo, Consolas, monospace;
  }
  @media (prefers-color-scheme: dark) {
    :root:not([data-theme="light"]) {
      color-scheme: dark;
      --bg: #0F1215; --surface: #161A1F; --sunk: #1D2229; --ink: #E6E9ED; --ink-2: #ADB5BF; --ink-3: #7E8792;
      --rule: #29303A; --rule-2: #3A424D; --accent: #8DA6FF; --accent-soft: #1A2348;
      --add: #6ED39A; --add-bg: #12281B; --del: #F28E81; --del-bg: #341A19; --hl: #E8B95E; --hl-bg: #2F2611;
      --code-bg: #12161B; --tk-key: #92ABFF; --tk-str: #5DCFC6; --tk-num: #E8B95E; --tk-bool: #E88CC1; --tk-com: #7E8792;
    }
  }
  :root[data-theme="dark"] {
    color-scheme: dark;
    --bg: #0F1215; --surface: #161A1F; --sunk: #1D2229; --ink: #E6E9ED; --ink-2: #ADB5BF; --ink-3: #7E8792;
    --rule: #29303A; --rule-2: #3A424D; --accent: #8DA6FF; --accent-soft: #1A2348;
    --add: #6ED39A; --add-bg: #12281B; --del: #F28E81; --del-bg: #341A19; --hl: #E8B95E; --hl-bg: #2F2611;
    --code-bg: #12161B; --tk-key: #92ABFF; --tk-str: #5DCFC6; --tk-num: #E8B95E; --tk-bool: #E88CC1; --tk-com: #7E8792;
  }

  * { box-sizing: border-box; }
  body { background: var(--bg); color: var(--ink); font: 16.5px/1.62 var(--body); -webkit-font-smoothing: antialiased; }
  a { color: var(--accent); text-underline-offset: 2px; }
  button { font: inherit; color: inherit; }
  :focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 3px; }
  code { font-family: var(--mono); font-size: 0.86em; background: var(--sunk); padding: 0.08em 0.36em; border-radius: 3px; word-break: break-word; }

  /* top bar */
  .top { position: sticky; top: env(safe-area-inset-top, 0px); z-index: 5; background: var(--bg); border-bottom: 1px solid var(--rule); }
  .top-in { max-width: 1240px; margin: 0 auto; padding: 12px 20px; display: flex; align-items: center; gap: 18px; flex-wrap: wrap; }
  .brand { font-family: var(--display); font-weight: 700; font-size: 18px; letter-spacing: -0.01em; display: flex; gap: 10px; align-items: baseline; }
  .brand small { font-family: var(--mono); font-weight: 400; font-size: 12px; color: var(--ink-3); }
  .prog { margin-left: auto; display: flex; align-items: center; gap: 10px; font-size: 13px; color: var(--ink-2); font-variant-numeric: tabular-nums; }
  .here { margin-left: auto; border: 1px solid var(--rule-2); background: var(--surface); border-radius: 6px; padding: 5px 11px; font-size: 13px; cursor: pointer; color: var(--ink-2); }
  .here:hover { color: var(--ink); border-color: var(--ink-3); }
  .here + .prog { margin-left: 0; }
  .bar { width: 120px; height: 6px; background: var(--sunk); border-radius: 3px; overflow: hidden; }
  .bar i { display: block; height: 100%; background: var(--add); width: 0; transition: width .3s; }
  .pick { display: none; width: 100%; }
  .pick select { width: 100%; padding: 8px 10px; border: 1px solid var(--rule-2); border-radius: 6px; background: var(--surface); color: var(--ink); font: 15px var(--body); }

  /* shell */
  .shell { max-width: 1240px; margin: 0 auto; padding-inline: 20px; padding-block: 28px 80px; display: grid; grid-template-columns: 268px minmax(0, 1fr); gap: 44px; }
  .rail { position: sticky; top: 76px; align-self: start; max-height: calc(100vh - 96px); overflow: auto; }
  .rail h2 { font: 600 11.5px var(--body); letter-spacing: .09em; text-transform: uppercase; color: var(--ink-3); margin: 0 0 10px; }
  .rail ol { list-style: none; margin: 0; padding: 0; }
  .rail button { width: 100%; text-align: left; background: none; border: 0; border-radius: 6px; padding: 7px 9px; display: grid; grid-template-columns: 22px 1fr 16px; gap: 8px; align-items: baseline; cursor: pointer; color: var(--ink-2); font-size: 14.5px; line-height: 1.35; }
  .rail button:hover { background: var(--sunk); color: var(--ink); }
  .rail button[aria-current="page"] { background: var(--surface); color: var(--ink); box-shadow: 0 0 0 1px var(--rule); font-weight: 500; }
  .rail .n { font-family: var(--mono); font-size: 12px; color: var(--ink-3); }
  .rail .ok { color: var(--add); font-size: 13px; }
  .rail .dot { display: inline-block; width: 7px; height: 7px; border-radius: 50%; margin-right: 6px; vertical-align: 1px; }
  .dot.breaking { background: var(--del); } .dot.quiet { background: var(--hl); } .dot.new { background: var(--add); }
  .dot.deprecated { background: var(--ink-3); } .dot.orientation { background: var(--accent); }
  .legend { margin-top: 18px; padding-top: 14px; border-top: 1px solid var(--rule); font-size: 12.5px; color: var(--ink-3); display: grid; gap: 4px; }

  @media (max-width: 979px) {
    .shell { grid-template-columns: minmax(0, 1fr); gap: 0; padding-top: 18px; }
    .rail { display: none; }
    .pick { display: block; }
    .prog { margin-left: 0; }
  }

  /* lesson */
  main { min-width: 0; max-width: 820px; }
  .kick { display: flex; flex-wrap: wrap; gap: 8px; align-items: center; margin-bottom: 14px; }
  .chip { font: 600 12px var(--body); letter-spacing: .07em; text-transform: uppercase; padding: 3px 9px; border-radius: 99px; }
  .chip.breaking { background: var(--del-bg); color: var(--del); }
  .chip.quiet { background: var(--hl-bg); color: var(--hl); }
  .chip.new { background: var(--add-bg); color: var(--add); }
  .chip.deprecated { background: var(--sunk); color: var(--ink-2); }
  .chip.orientation { background: var(--accent-soft); color: var(--accent); }
  .chip.domain { background: transparent; color: var(--ink-3); box-shadow: inset 0 0 0 1px var(--rule-2); }
  .pos { font-family: var(--mono); font-size: 12px; color: var(--ink-3); margin-left: auto; }
  h1 { font-family: var(--display); font-weight: 700; font-size: clamp(30px, 5vw, 44px); line-height: 1.08; letter-spacing: -0.02em; margin: 0 0 14px; text-wrap: balance; }
  .lede { font-size: 19px; line-height: 1.5; color: var(--ink-2); max-width: 62ch; margin: 0; }

  .sec { margin-top: 44px; }
  .sec-h { display: flex; align-items: baseline; gap: 10px; margin: 0 0 14px; font: 600 12px var(--body); letter-spacing: .09em; text-transform: uppercase; color: var(--ink-3); }
  .sec-h::after { content: ""; flex: 1; height: 1px; background: var(--rule); }
  .prose p, .prose li { max-width: 66ch; }
  .prose p { margin: 0 0 0.9em; }
  .prose ul, .prose ol { padding-left: 1.2em; margin: 0 0 1em; }
  .prose li { margin-bottom: 0.45em; }
  .prose li::marker { color: var(--ink-3); }
  .prose > :last-child { margin-bottom: 0; }

  /* figures */
  .fig { background: var(--surface); border: 1px solid var(--rule); border-radius: 8px; overflow: hidden; }
  .fig + .fig { margin-top: 16px; }
  .fig-bar { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; padding: 9px 12px; border-bottom: 1px solid var(--rule); }
  .fig-bar strong { font-size: 14px; font-weight: 600; }
  .seg { margin-left: auto; display: inline-flex; background: var(--sunk); border-radius: 6px; padding: 2px; }
  .seg button { border: 0; background: none; padding: 4px 11px; border-radius: 5px; font-size: 12.5px; cursor: pointer; color: var(--ink-2); }
  .seg button[aria-pressed="true"] { background: var(--surface); color: var(--ink); box-shadow: 0 0 0 1px var(--rule); font-weight: 500; }
  .fig-scroll { overflow-x: auto; padding: 8px 6px 4px; }
  .fig-cap { padding: 0 14px 12px; font-size: 13.5px; color: var(--ink-2); }

  svg.seq { display: block; width: 100%; min-width: 600px; height: auto; font-family: var(--mono); }
  .sq-life { stroke: var(--rule-2); stroke-dasharray: 3 4; }
  .sq-act { fill: var(--sunk); stroke: var(--rule-2); }
  .sq-at { fill: var(--ink); font-size: 12.5px; font-weight: 500; font-family: var(--body); }
  .sq-lb { fill: var(--ink); font-size: 11.5px; }
  .sq-ln { stroke: var(--ink-2); stroke-width: 1.4; }
  .sq-ln.dash { stroke-dasharray: 5 4; }
  .sq-hd { fill: var(--ink-2); }
  .sq-note { fill: var(--hl-bg); stroke: none; }
  .sq-nt { fill: var(--ink-2); font-size: 12px; font-family: var(--body); }
  .sq-lb, .sq-nt { paint-order: stroke; stroke: var(--surface); stroke-width: 4px; stroke-linejoin: round; }
  .sq-lb.del { fill: var(--del); }
  .sq-ln.del { stroke: var(--del); } .sq-hd.del { fill: var(--del); }
  .sq-lb.add { fill: var(--add); font-weight: 500; }
  .sq-ln.add { stroke: var(--add); } .sq-hd.add { fill: var(--add); }
  .sq-lb.err { fill: var(--del); }
  .sq-ln.err { stroke: var(--del); } .sq-hd.err { fill: var(--del); }
  .sq-note.add { fill: var(--add-bg); } .sq-note.del { fill: var(--del-bg); } .sq-note.acc { fill: var(--accent-soft); }

  /* code */
  .pay + .pay { margin-top: 22px; }
  .pay-t { font-weight: 600; font-size: 15px; margin: 0 0 8px; }
  .code-wrap { border: 1px solid var(--rule); border-radius: 8px; overflow: hidden; background: var(--code-bg); }
  .code-wrap + .code-wrap { margin-top: 10px; }
  .code-head { display: flex; align-items: center; gap: 8px; padding: 6px 8px 6px 12px; border-bottom: 1px solid var(--rule); background: var(--surface); font-size: 12.5px; color: var(--ink-2); }
  .code-head .era { font: 600 11.5px var(--body); letter-spacing: .08em; text-transform: uppercase; padding: 2px 7px; border-radius: 3px; }
  .era.legacy { background: var(--del-bg); color: var(--del); }
  .era.modern { background: var(--add-bg); color: var(--add); }
  .era.shell { background: var(--accent-soft); color: var(--accent); }
  .copy { margin-left: auto; border: 1px solid var(--rule-2); background: var(--surface); border-radius: 5px; padding: 2px 9px; font-size: 12px; cursor: pointer; color: var(--ink-2); }
  .copy:hover { color: var(--ink); border-color: var(--ink-3); }
  pre.code { margin: 0; padding: 10px 0; overflow-x: auto; font: 13px/1.62 var(--mono); }
  .ln { display: block; padding: 0 16px; white-space: pre; min-height: 1.62em; }
  .ln.add { background: var(--add-bg); box-shadow: inset 3px 0 0 var(--add); }
  .ln.del { background: var(--del-bg); box-shadow: inset 3px 0 0 var(--del); }
  .ln.del .tk-k, .ln.del .tk-s, .ln.del .tk-n, .ln.del .tk-b, .ln.del .tk-h, .ln.del .tk-m { text-decoration: line-through; text-decoration-color: color-mix(in srgb, var(--del) 55%, transparent); }
  .ln.hl { background: var(--hl-bg); box-shadow: inset 3px 0 0 var(--hl); }
  .tk-k { color: var(--tk-key); } .tk-s { color: var(--tk-str); } .tk-n { color: var(--tk-num); } .tk-b { color: var(--tk-bool); }
  .tk-c { color: var(--tk-com); font-style: italic; } .tk-h { color: var(--tk-key); font-weight: 500; } .tk-m { color: var(--tk-bool); font-weight: 500; }
  .pay-cap { font-size: 13.5px; color: var(--ink-2); margin: 8px 0 0; max-width: 66ch; }

  /* callouts */
  .call { border-radius: 8px; padding: 18px 20px; }
  .call.impact { background: var(--surface); border: 1px solid var(--rule); border-left: 4px solid var(--del); }
  .call.excite { background: var(--add-bg); }
  .call.why { background: var(--surface); border: 1px solid var(--rule); }
  .expect { margin-top: 12px; font-size: 14.5px; color: var(--ink-2); }

  /* quiz */
  .q { background: var(--surface); border: 1px solid var(--rule); border-radius: 8px; padding: 18px 20px; }
  .q + .q { margin-top: 14px; }
  .q-t { font-weight: 600; margin: 0 0 12px; max-width: 64ch; }
  .opts { display: grid; gap: 8px; }
  .opt { text-align: left; border: 1px solid var(--rule-2); background: var(--bg); border-radius: 6px; padding: 9px 12px; cursor: pointer; font-size: 15px; line-height: 1.4; }
  .opt:hover:not(:disabled) { border-color: var(--accent); }
  .opt:disabled { cursor: default; }
  .opt.right { border-color: var(--add); background: var(--add-bg); color: var(--ink); }
  .opt.wrong { border-color: var(--del); background: var(--del-bg); }
  .q-x { margin: 12px 0 0; font-size: 14.5px; color: var(--ink-2); max-width: 64ch; }
  .q-x b { color: var(--ink); }
  .q-reset { margin-top: 10px; border: 0; background: none; color: var(--accent); cursor: pointer; font-size: 13px; padding: 0; text-decoration: underline; }

  /* links + nav */
  .links { display: flex; flex-wrap: wrap; gap: 8px 16px; font-size: 14px; }
  .nav { margin-top: 52px; padding-top: 20px; border-top: 2px solid var(--ink); display: grid; grid-template-columns: 1fr auto 1fr; gap: 12px; align-items: center; }
  .nav .btn { border: 1px solid var(--rule-2); background: var(--surface); border-radius: 7px; padding: 10px 14px; cursor: pointer; font-size: 14.5px; line-height: 1.3; }
  .nav .btn:hover { border-color: var(--ink-3); }
    .nav .prev { justify-self: start; text-align: left; } .nav .next { justify-self: end; text-align: right; }
  .nav small { display: block; color: var(--ink-3); font-size: 12px; }
  .btn { display: inline-block; text-decoration: none; color: var(--ink); }
  .learn { background: var(--ink) !important; color: var(--bg); border-color: var(--ink) !important; font-weight: 500; }
  .learn.done { background: var(--add-bg) !important; color: var(--add); border-color: var(--add) !important; }
  .kbd { font-size: 12.5px; color: var(--ink-3); margin-top: 12px; text-align: center; }
  @media (max-width: 600px) {
    .nav { grid-template-columns: 1fr 1fr; }
    .nav .learn { grid-column: 1 / -1; grid-row: 1; }
    .kbd { display: none; }
  }

  .grid2 { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 14px; }
  .card { background: var(--surface); border: 1px solid var(--rule); border-radius: 8px; padding: 16px 18px; }
  .card h3 { font-family: var(--display); font-size: 18px; margin: 0 0 6px; }
  .card p { margin: 0; font-size: 15px; color: var(--ink-2); }
  .swatches { display: grid; gap: 6px; font: 13px var(--mono); }
  .swatches span { padding: 4px 12px; border-radius: 4px; }

  /* deep links */
  .sec, .pay, .q, .code-wrap, h1 { scroll-margin-top: 88px; }
  .sec-h a.hl, .pay-t a.hl { color: inherit; text-decoration: none; }
  .sec-h a.hl:hover, .pay-t a.hl:hover { color: var(--accent); }
  .lnk { border: 0; background: none; padding: 0 4px; margin: 0; cursor: pointer; color: var(--ink-3); font: 500 13px var(--mono); opacity: 0; transition: opacity .15s; letter-spacing: 0; text-transform: none; }
  .sec-h:hover .lnk, .pay-t:hover .lnk, .q-t:hover .lnk, .code-head:hover .lnk, .lnk:focus-visible, .lnk.copied { opacity: 1; }
  .lnk:hover { color: var(--accent); }
  .lnk.copied { color: var(--add); }
  @media (hover: none) { .lnk { opacity: .55; } }
  .flash { animation: flash 1.6s ease-out; }
  @keyframes flash { 0%, 30% { box-shadow: 0 0 0 3px var(--accent-soft), 0 0 0 4px var(--accent); border-radius: 6px; } 100% { box-shadow: 0 0 0 3px transparent; } }
  .rail .toc { list-style: none; margin: 2px 0 8px 31px; padding: 0 0 0 10px; border-left: 1px solid var(--rule-2); }
  .rail .toc a { display: block; padding: 3px 6px; font-size: 13px; color: var(--ink-3); text-decoration: none; border-radius: 4px; line-height: 1.35; }
  .rail .toc a:hover { color: var(--ink); background: var(--sunk); }
  .rail .toc a.on { color: var(--accent); }
  .rail a.lesson { width: 100%; text-align: left; border-radius: 6px; padding: 7px 9px; display: grid; grid-template-columns: 22px 1fr 16px; gap: 8px; align-items: baseline; color: var(--ink-2); font-size: 14.5px; line-height: 1.35; text-decoration: none; }
  .rail a.lesson:hover { background: var(--sunk); color: var(--ink); }
  .rail a.lesson[aria-current="page"] { background: var(--surface); color: var(--ink); box-shadow: 0 0 0 1px var(--rule); font-weight: 500; }
  .toast { position: fixed; left: 50%; bottom: calc(18px + env(safe-area-inset-bottom, 0px)); transform: translateX(-50%); background: var(--ink); color: var(--bg); font-size: 13.5px; padding: 8px 14px; border-radius: 7px; max-width: calc(100vw - 32px); overflow-wrap: anywhere; z-index: 9; }

  @media (prefers-reduced-motion: reduce) { * { transition: none !important; animation: none !important; } }

  /* real-site additions */
  .brand { color: var(--ink); text-decoration: none; }
  .rail h2 a { color: inherit; text-decoration: none; }
  .sec:target, .pay:target, .q:target, .code-wrap:target { animation: flash 1.6s ease-out; }
  .home { max-width: 1100px; margin: 0 auto; padding-inline: 20px; padding-block: 44px 80px; }
  .eyebrow { font: 500 13px var(--mono); color: var(--ink-3); margin: 0 0 14px; }
  .home h1 { font-size: clamp(34px, 6vw, 60px); max-width: 18ch; }
  .cta { margin: 26px 0 40px; display: flex; flex-wrap: wrap; gap: 14px; align-items: center; }
  .cta .btn { border: 1px solid var(--ink); border-radius: 7px; padding: 11px 18px; font-size: 15px; }
  .resume { font-size: 15px; color: var(--ink-2); }
  .lesson-grid { list-style: none; padding: 0; margin: 0; display: grid; grid-template-columns: repeat(auto-fill, minmax(290px, 1fr)); gap: 14px; }
  .lcard { display: flex; flex-direction: column; gap: 8px; height: 100%; background: var(--surface); border: 1px solid var(--rule); border-radius: 8px; padding: 16px 18px; color: var(--ink); text-decoration: none; }
  .lcard:hover { border-color: var(--accent); }
  .lc-top { display: flex; align-items: center; gap: 10px; }
  .lc-top .n { font: 12px var(--mono); color: var(--ink-3); }
  .lc-top .ok { margin-left: auto; color: var(--add); }
  .lcard strong { font-family: var(--display); font-size: 19px; line-height: 1.2; }
  .lc-d { font-size: 14.5px; color: var(--ink-2); line-height: 1.5; }
  .foot { margin-top: 44px; font-size: 14px; color: var(--ink-3); max-width: 70ch; }

  /* tier 2: readability, accessibility, engagement */
  :root { --control: #858C83; }
  @media (prefers-color-scheme: dark) { :root:not([data-theme="light"]) { --control: #737C88; } }
  :root[data-theme="dark"] { --control: #737C88; }
  .pick select, .opt, .copy, .nav .btn, .here, .seg, .cta .btn { border-color: var(--control); }
  .seg { box-shadow: inset 0 0 0 1px var(--control); }
  pre.code code { display: block; font-size: inherit; background: none; padding: 0; border-radius: 0; word-break: normal; }
  pre.code:focus-visible, .fig-scroll:focus-visible { outline: 2px solid var(--accent); outline-offset: -2px; }
  .sr-only { position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0; }
  .skip { position: absolute; left: 12px; top: -60px; z-index: 10; background: var(--ink); color: var(--bg); padding: 8px 14px; border-radius: 6px; text-decoration: none; }
  .skip:focus { top: 10px; }
  .sec-h h2 { font: inherit; letter-spacing: inherit; text-transform: inherit; color: inherit; margin: 0; }

  /* diagram toggle + scroll hint */
  .seg-wrap { margin-left: auto; display: flex; align-items: center; gap: 8px; }
  .seg-wrap .seg { margin-left: 0; }
  .seg-l { font: 600 11.5px var(--body); letter-spacing: .08em; text-transform: uppercase; color: var(--ink-3); }
  .seg button { font-size: 13px; padding: 5px 12px; }
  .fig-prompt { margin: 0; padding: 10px 14px 0; font-size: 14px; color: var(--ink-2); }
  .fig-scroll { position: relative; }
  @media (max-width: 700px) {
    .fig-scroll { -webkit-mask-image: linear-gradient(to right, #000 85%, transparent); mask-image: linear-gradient(to right, #000 85%, transparent); }
    .fig-cap::before { content: "Scroll sideways to see the whole diagram. "; font-weight: 500; }
  }

  /* exam TL;DR */
  .tldr { margin-top: 24px; background: var(--accent-soft); border-radius: 8px; padding: 14px 18px 14px; max-width: 70ch; }
  .tldr-h { font: 600 12px var(--body); letter-spacing: .09em; text-transform: uppercase; color: var(--accent); margin: 0 0 6px; }
  .tldr ul { margin: 0; padding-left: 1.1em; }
  .tldr li { font-size: 15.5px; line-height: 1.5; margin: 3px 0; }
  .tldr li::marker { color: var(--accent); }

  /* real-life story */
  .story { background: var(--surface); border: 1px solid var(--rule); border-radius: 8px; padding: 20px 22px; position: relative; }
  .story::before { content: ""; position: absolute; inset: 0 auto 0 0; width: 4px; border-radius: 8px 0 0 8px; background: linear-gradient(var(--hl), var(--add)); }
  .story-t { font-family: var(--display); font-weight: 700; font-size: 22px; line-height: 1.2; margin: 0 0 10px; }
  .story .prose p { font-size: 16.5px; }

  /* test setup */
  .setup { margin-bottom: 14px; border: 1px dashed var(--control); border-radius: 8px; padding: 0 12px; }
  .setup summary { cursor: pointer; padding: 10px 0; font-size: 14.5px; font-weight: 500; color: var(--accent); }
  .setup[open] { padding-bottom: 12px; }

  /* quiz */
  .quiz-note { font-size: 14px; color: var(--ink-2); margin: 0 0 12px; }
  .q-n { font: 600 12px var(--mono); color: var(--accent); margin-right: 8px; }
  .opt { display: flex; gap: 10px; align-items: baseline; }
  .opt-l { font: 600 12.5px var(--mono); color: var(--ink-3); flex: none; }
  .opt.right .opt-l { color: var(--add); } .opt.wrong .opt-l { color: var(--del); }

  /* phone: header scrolls away so anchors land on their heading */
  @media (max-width: 979px) {
    .top { position: static; }
    .here, .brand small { display: none; }
    .top-in { gap: 10px 14px; }
    .sec, .pay, .q, .code-wrap, h1 { scroll-margin-top: 16px; }
  }

  /* practice: top links, exam, review */
  .toplinks { display: flex; gap: 14px; margin-left: auto; font-size: 14px; }
  .toplinks a { color: var(--ink-2); text-decoration: none; display: inline-flex; align-items: center; gap: 6px; }
  .toplinks a:hover, .toplinks a[aria-current="page"] { color: var(--accent); }
  .toplinks + .here { margin-left: 0; }
  .due { background: var(--del); color: #fff; font: 600 11px var(--body); border-radius: 99px; padding: 1px 7px; }
  .rail .ok { font: 500 11.5px var(--mono); white-space: nowrap; }
  .practice { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 14px; margin: 0 0 40px; }
  .pcard { display: flex; flex-direction: column; gap: 6px; padding: 16px 18px; border-radius: 8px; background: var(--accent-soft); color: var(--ink); text-decoration: none; border: 1px solid transparent; }
  .pcard:hover { border-color: var(--accent); }
  .pcard strong { font-family: var(--display); font-size: 19px; }
  .pcard span { font-size: 14.5px; color: var(--ink-2); line-height: 1.5; }
  .practice-app { max-width: 760px; margin-top: 20px; }
  .pq { background: var(--surface); border: 1px solid var(--rule); border-radius: 10px; padding: 20px 22px; }
  .pq-meta { display: flex; justify-content: space-between; gap: 12px; margin: 0 0 10px; font-size: 13px; color: var(--ink-3); }
  .pq-meta a { color: var(--ink-3); }
  .pq-bar { height: 4px; background: var(--sunk); border-radius: 2px; margin-bottom: 16px; overflow: hidden; }
  .pq-bar i { display: block; height: 100%; background: var(--accent); transition: width .3s; }
  .pq .q-t { font-size: 18px; }
  .pq-next { margin: 16px 0 0; }
  .pq-next .btn, .cta button.btn { border: 1px solid var(--ink); border-radius: 7px; padding: 10px 18px; cursor: pointer; font-size: 15px; }
  .cta .btn:not(.learn) { background: var(--surface); }
  .q-x p { margin: 0 0 8px; }
  .why-not { margin-top: 8px; font-size: 14px; }
  .why-not summary { cursor: pointer; color: var(--accent); }
  .why-not ul { margin: 8px 0 0; padding-left: 1.1em; }
  .why-not li { margin-bottom: 6px; color: var(--ink-2); }
  .wn-o { color: var(--ink); font-weight: 500; }
  .wn-o::after { content: " —"; color: var(--ink-3); }
  .dom-plan, .dom-bars, .missed { list-style: none; padding: 0; margin: 0 0 20px; }
  .dom-plan li { display: flex; justify-content: space-between; padding: 8px 0; border-bottom: 1px solid var(--rule); font-size: 15px; }
  .dom-plan b { font-weight: 500; color: var(--ink-2); font-variant-numeric: tabular-nums; }
  .last { color: var(--ink-2); }
  .result { display: flex; gap: 24px; align-items: center; padding: 22px 24px; border-radius: 10px; margin-bottom: 28px; }
  .result.ready { background: var(--add-bg); } .result.close { background: var(--hl-bg); } .result.not-yet { background: var(--del-bg); }
  .score { font-family: var(--display); font-size: 64px; font-weight: 700; line-height: 1; margin: 0; font-variant-numeric: tabular-nums; }
  .score small { font-size: 28px; }
  .verdict { font-family: var(--display); font-size: 22px; font-weight: 700; margin: 0 0 4px; }
  .result p:last-child { margin: 0; }
  h2.sub { font: 600 12px var(--body); letter-spacing: .09em; text-transform: uppercase; color: var(--ink-3); margin: 26px 0 10px; }
  .dom-bars li { display: grid; grid-template-columns: minmax(0, 1fr) 140px 44px; gap: 12px; align-items: center; padding: 7px 0; font-size: 15px; }
  .dom-bars small { color: var(--ink-3); }
  .db { height: 8px; background: var(--sunk); border-radius: 4px; overflow: hidden; }
  .db i { display: block; height: 100%; background: var(--accent); }
  .dom-bars b { text-align: right; font-variant-numeric: tabular-nums; font-weight: 500; }
  .missed li { padding: 8px 0; border-bottom: 1px solid var(--rule); font-size: 15px; }
  .missed a { font-weight: 500; margin-right: 6px; }
  .note-sm { font-size: 14px; color: var(--ink-2); }
  .boxes { display: flex; gap: 5px; margin: 0 0 10px; }
  .boxes span { width: 22px; height: 6px; border-radius: 3px; background: var(--sunk); }
  .boxes span.on { background: var(--add); }
  @media (max-width: 600px) { .dom-bars li { grid-template-columns: 1fr 70px 40px; } .result { flex-direction: column; align-items: flex-start; } .toplinks { margin-left: 0; } }

  /* two tracks */
  .rail-track { font: 600 11.5px var(--body); letter-spacing: .08em; text-transform: uppercase; color: var(--ink-3); margin: 0 0 10px; }
  .rail-other { margin: 14px 0 0; font-size: 13.5px; }
  .rail-other a { color: var(--accent); text-decoration: none; }
  .era.bad { background: var(--del-bg); color: var(--del); }
  .era.good { background: var(--add-bg); color: var(--add); }
  .era.ok { background: var(--sunk); color: var(--ink-2); }
  .era.err { background: var(--hl-bg); color: var(--hl); }
  .track { margin-top: 48px; }
  .track-h { max-width: 70ch; margin-bottom: 16px; }
  .track-h .eyebrow { margin-bottom: 6px; }
  .track-h h2 { font-family: var(--display); font-size: clamp(24px, 3.5vw, 32px); margin: 0 0 6px; letter-spacing: -0.01em; }
  .track-h p:last-child { color: var(--ink-2); margin: 0; }
  .cta .btn:not(.learn) { border: 1px solid var(--control); border-radius: 7px; padding: 11px 18px; font-size: 15px; }

  /* attack table */
  .prose table.atk { width: 100%; border-collapse: collapse; font-size: 14.5px; margin: 6px 0 14px; display: block; overflow-x: auto; }
  .atk th { text-align: left; font: 600 11.5px var(--body); letter-spacing: .06em; text-transform: uppercase; color: var(--ink-3); padding: 8px 10px; border-bottom: 1.5px solid var(--rule-2); }
  .atk td { padding: 10px; border-bottom: 1px solid var(--rule); vertical-align: top; min-width: 160px; line-height: 1.45; }

  /* legacy content folded away */
  .legacy-fold { margin-top: 10px; border: 1px dashed var(--control); border-radius: 8px; padding: 0 12px; }
  .legacy-fold summary { cursor: pointer; padding: 9px 0; font-size: 13.5px; color: var(--ink-2); }
  .legacy-fold[open] { padding-bottom: 12px; }
  .legacy-fold .code-wrap { opacity: .9; }
  .order-note { max-width: 70ch; font-size: 14.5px; color: var(--ink-2); margin: -20px 0 32px; }

  /* listen button */
  .listen { margin: 18px 0 0; }
  .listen-btn { display: inline-flex; align-items: center; gap: 12px; padding: 10px 16px 10px 12px; border-radius: 10px; background: var(--surface); border: 1px solid var(--control); color: var(--ink); text-decoration: none; }
  .listen-btn:hover { border-color: var(--accent); }
  .listen-ico { font-size: 24px; line-height: 1; }
  .listen-btn strong { display: block; font-size: 15px; }
  .listen-btn small { display: block; font-size: 12.5px; color: var(--ink-2); }
  .lc-audio { font-size: 14px; }

  /* feedback line */
  .fb-line { margin: 40px 0 0; padding: 12px 16px; border-radius: 8px; background: var(--sunk); font-size: 14.5px; color: var(--ink-2); max-width: 70ch; }
  .fb-line a { font-weight: 500; }
  .home .fb-line { margin-top: 18px; }

  /* progress, sync, resume, storage warning */
  .store-warn { background: var(--del-bg); color: var(--ink); border-bottom: 1px solid var(--del); padding: 10px 20px; font-size: 14.5px; text-align: center; }
  .resume-box { background: var(--hl-bg); border-radius: 10px; padding: 16px 20px; margin-bottom: 20px; }
  .resume-box p { margin: 0 0 8px; }
  .overall { margin: 4px 0 20px; max-width: 760px; }
  .overall-head { display: flex; align-items: baseline; gap: 8px; margin-bottom: 8px; }
  .overall-head b { font-family: var(--display); font-size: 34px; line-height: 1; font-variant-numeric: tabular-nums; }
  .overall-head span { color: var(--ink-2); font-size: 14.5px; }
  .overall-bar { height: 12px; background: var(--sunk); border-radius: 6px; overflow: hidden; }
  .overall-bar i { display: block; height: 100%; background: var(--add); border-radius: 6px; transition: width .4s ease; }
  .overall .note-sm { margin: 8px 0 0; }
  .tiles { display: grid; grid-template-columns: repeat(auto-fit, minmax(150px, 1fr)); gap: 12px; margin: 6px 0 24px; }
  .tile { background: var(--surface); border: 1px solid var(--rule); border-radius: 10px; padding: 14px 16px; display: flex; flex-direction: column; gap: 4px; }
  .tile b { font-family: var(--display); font-size: 30px; line-height: 1; font-variant-numeric: tabular-nums; }
  .tile b small { font-size: 16px; color: var(--ink-3); }
  .tile span { font-size: 13.5px; color: var(--ink-2); }
  h2.sub small { text-transform: none; letter-spacing: 0; font-weight: 400; }
  .attempts { width: 100%; border-collapse: collapse; font-size: 14px; font-variant-numeric: tabular-nums; display: block; overflow-x: auto; }
  .attempts th { text-align: left; font: 600 11.5px var(--body); letter-spacing: .06em; color: var(--ink-3); padding: 6px 10px 6px 0; border-bottom: 1.5px solid var(--rule-2); }
  .attempts td { padding: 8px 10px 8px 0; border-bottom: 1px solid var(--rule); white-space: nowrap; }
  #attempt-detail { margin-top: 18px; }
  .linkish { background: none; border: 0; padding: 0; color: var(--accent); cursor: pointer; text-decoration: underline; font-size: 13.5px; }
  .lesson-scores { list-style: none; padding: 0; margin: 0 0 18px; }
  .lesson-scores li { display: grid; grid-template-columns: minmax(0, 1fr) 110px 90px 18px; gap: 12px; align-items: center; padding: 7px 0; border-bottom: 1px solid var(--rule); font-size: 14.5px; }
  .ls-bar { height: 6px; background: var(--sunk); border-radius: 3px; overflow: hidden; }
  .ls-bar i { display: block; height: 100%; background: var(--add); }
  .ls-n { font-variant-numeric: tabular-nums; color: var(--ink-2); }
  .ls-ok { color: var(--add); }
  .sync-panel { background: var(--surface); border: 1px solid var(--rule); border-radius: 10px; padding: 18px 20px; max-width: 760px; }
  .sync-panel p { margin: 0 0 10px; }
  .sync-code { font: 600 24px var(--mono); letter-spacing: .06em; color: var(--accent); }
  .sync-err { color: var(--del); }
  .sync-join { margin-top: 14px; }
  .sync-join label { display: block; font-size: 14px; font-weight: 500; margin-bottom: 6px; }
  .sync-join div { display: flex; gap: 8px; flex-wrap: wrap; }
  .sync-join input { font: 15px var(--mono); padding: 9px 12px; border: 1px solid var(--control); border-radius: 7px; background: var(--bg); color: var(--ink); min-width: 0; flex: 1 1 200px; }
  .sync-join .btn, .sync-panel .btn, .cta button.btn { border: 1px solid var(--control); border-radius: 7px; padding: 9px 16px; cursor: pointer; font-size: 14.5px; background: var(--surface); color: var(--ink); }
  .sync-panel .btn.learn { background: var(--ink); color: var(--bg); border-color: var(--ink); }
  .backup-box { width: 100%; min-height: 110px; font: 12.5px var(--mono); padding: 10px; border-radius: 8px; border: 1px solid var(--control); background: var(--bg); color: var(--ink); }
  @media (max-width: 600px) { .lesson-scores li { grid-template-columns: minmax(0, 1fr) 60px 70px 16px; } }