:root{
  --ink:#0f172a;--muted:#5b6b80;--line:#e2e8f0;--paper:#fff;--wash:#f6f8fc;
  --navy:#123f8c;--navy-deep:#0d2f6b;--blue:#2563eb;--blue-soft:#eff6ff;
  --amber-bg:#fff4d6;--amber-line:#e6c383;--amber-ink:#7a520c;
  --radius:16px;--shadow:0 1px 2px #0f172a0a,0 8px 24px #0f172a08;
}
*{box-sizing:border-box}
html{scroll-behavior:smooth;-webkit-text-size-adjust:100%}
body{
  margin:0;background:var(--wash);color:var(--ink);
  font:17px/1.7 Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",sans-serif;
  font-feature-settings:"rlig" 1,"calt" 1;
}
h1,h2,h3,h4{letter-spacing:-.02em;line-height:1.2;margin:0 0 12px}
h1{font-size:clamp(28px,4.4vw,42px)}
h2{font-size:clamp(21px,2.6vw,27px)}
h3{font-size:19px}
p{margin:0 0 14px;overflow-wrap:break-word}
a{color:#1d4ed8;text-underline-offset:3px}
img,svg{max-width:100%}
:focus-visible{outline:3px solid #b45309;outline-offset:3px;border-radius:6px}
.shell{max-width:1140px;margin:0 auto;padding:0 24px;width:100%}
[id]{scroll-margin-top:92px}
.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}

/* ---------- header ---------- */
.site-header{
  position:sticky;top:0;z-index:40;background:#fffffff2;backdrop-filter:blur(10px);
  border-bottom:1px solid var(--line);
}
.site-header .shell{display:flex;align-items:center;gap:18px;min-height:66px;flex-wrap:wrap;padding-top:10px;padding-bottom:10px}
.brand{display:flex;align-items:center;gap:10px;font-weight:800;font-size:19px;color:var(--ink);text-decoration:none;letter-spacing:-.02em}
.brand-mark{border-radius:9px;flex:none}
.brand span i{font-style:normal;color:var(--navy)}
.brand small{display:block;font-size:11px;font-weight:700;letter-spacing:.14em;text-transform:uppercase;color:var(--muted)}
.nav-pills{display:flex;gap:4px;background:#f1f5f9;border-radius:999px;padding:4px;margin-left:auto;flex-wrap:wrap}
.nav-pills a{
  display:inline-block;padding:8px 16px;border-radius:999px;text-decoration:none;
  font-size:15px;font-weight:600;color:#33465e;white-space:nowrap;
}
.nav-pills a:hover{background:#e2e8f0;color:var(--ink)}
.nav-pills a[aria-current=page]{background:var(--navy);color:#fff}
.draft-flag{
  font-size:12.5px;font-weight:700;color:var(--amber-ink);background:var(--amber-bg);
  border:1px solid var(--amber-line);border-radius:999px;padding:5px 12px;white-space:nowrap;
}

/* ---------- breadcrumb strip ---------- */
.crumb-bar{background:#fff;border-bottom:1px solid var(--line)}
.crumb-bar .shell{display:flex;align-items:center;gap:14px;flex-wrap:wrap;padding-top:11px;padding-bottom:11px}
.breadcrumbs{display:flex;align-items:center;gap:8px;flex-wrap:wrap;font-size:14.5px;color:var(--muted);margin:0}
.breadcrumbs a{color:#41556e;text-decoration:none;font-weight:600}
.breadcrumbs a:hover{color:var(--blue);text-decoration:underline}
.breadcrumbs [aria-current=page]{color:var(--ink);font-weight:700}
.crumb-meta{margin-left:auto;display:flex;gap:16px;font-size:14px;color:var(--muted);font-weight:600}

/* ---------- hero ---------- */
.hero{background:linear-gradient(120deg,var(--navy-deep),var(--navy) 45%,#2a63c9);color:#fff}
.hero .shell{padding-top:38px;padding-bottom:38px}
.hero .eyebrow{
  display:inline-block;font-size:12.5px;font-weight:800;letter-spacing:.14em;text-transform:uppercase;
  background:#ffffff26;border:1px solid #ffffff33;border-radius:999px;padding:5px 14px;margin:0 0 14px;color:#e8eefc;
}
.hero h1{color:#fff;max-width:24ch}
.hero .lead{font-size:18.5px;color:#dbe6fb;max-width:70ch;margin:0}
.hero-stats{display:flex;gap:10px;flex-wrap:wrap;margin-top:18px}
.hero-stats span{
  background:#ffffff1f;border:1px solid #ffffff2e;border-radius:999px;
  padding:7px 15px;font-size:14.5px;font-weight:600;color:#fff;
}
main.shell{padding-top:32px;padding-bottom:56px}

/* ---------- lesson layout ---------- */
.lesson-layout{display:grid;grid-template-columns:248px minmax(0,1fr);gap:40px;align-items:start;padding-top:28px;padding-bottom:56px}
.toc{position:sticky;top:88px}
.toc h2{font-size:15px;text-transform:uppercase;letter-spacing:.1em;color:var(--muted);margin-bottom:12px}
.toc ol{list-style:none;margin:0;padding:0;border-left:2px solid var(--line)}
.toc li a{display:block;padding:9px 0 9px 16px;margin-left:-2px;border-left:2px solid transparent;color:#41556e;text-decoration:none;font-size:15px;font-weight:600}
.toc li a:hover{color:var(--blue);border-left-color:var(--blue);background:#f1f5fd}
.toc .toc-note{margin-top:18px;font-size:13.5px;color:var(--muted);background:#fff;border:1px solid var(--line);border-radius:12px;padding:14px}

/* ---------- lesson head, chips + standard callout ---------- */
.lesson-head{margin:0 0 6px}
.lesson-head h1{margin:4px 0 10px}
.lesson-head .meta{margin:0}
.chip-row{display:flex;align-items:center;gap:10px;flex-wrap:wrap;margin-bottom:14px;font-size:14.5px;color:var(--muted)}
.chip{
  display:inline-flex;align-items:center;gap:7px;font-weight:700;font-size:14px;
  border-radius:999px;padding:6px 13px;background:#eef2f9;color:#33465e;border:1px solid var(--line);
}
.chip[data-subject-chip]{background:#fff;border-color:currentColor}
.standard-callout{
  background:var(--blue-soft);border:1px solid #c7dbfb;border-radius:14px;
  padding:18px 20px;margin:20px 0 8px;font-size:15.5px;
}
.standard-callout .label{
  display:block;font-size:12px;font-weight:800;letter-spacing:.1em;text-transform:uppercase;
  color:#1e4ba8;margin-bottom:8px;
}
.standard-callout code{font-family:ui-monospace,SFMono-Regular,Menlo,monospace;font-weight:700;color:#123f8c;background:#fff;border:1px solid #cddcf7;border-radius:5px;padding:1px 6px;font-size:13.5px}
.standard-callout p{margin:0 0 8px}
.standard-callout p:last-child{margin:0}
.standard-callout .codes{display:flex;gap:8px;flex-wrap:wrap;margin:0 0 10px}
.standard-callout details summary{cursor:pointer;font-weight:700;color:#1e4ba8;font-size:14.5px}
.standard-callout details[open] summary{margin-bottom:10px}
.standard-callout details p{font-size:15px;color:#2c3d52}

/* ---------- section cards ---------- */
.section-card{background:var(--paper);border:1px solid var(--line);border-radius:var(--radius);box-shadow:var(--shadow);margin:22px 0;overflow:hidden}
.section-head{display:flex;align-items:center;gap:14px;padding:15px 22px;color:#fff;cursor:pointer;list-style:none;background:linear-gradient(100deg,var(--from),var(--to))}
.section-head::-webkit-details-marker{display:none}
.section-icon{font-size:22px;line-height:1;flex:none}
.section-title{flex:1;min-width:0}
.section-head .section-num{font-size:11.5px;font-weight:800;letter-spacing:.13em;text-transform:uppercase;color:#ffffffd9;display:block}
.section-head .section-h{display:block;font-size:22px;font-weight:800;letter-spacing:-.02em;line-height:1.25}
.section-toggle{flex:none;width:22px;height:22px;position:relative;transition:transform .18s}
.section-toggle::before{content:"";position:absolute;left:4px;top:6px;width:11px;height:11px;border-right:2.5px solid #fff;border-bottom:2.5px solid #fff;transform:rotate(45deg)}
.section-card[open] .section-toggle{transform:rotate(180deg)}
.section-head h2{margin:0;color:#fff;font-size:22px}
.section-body{padding:22px}
.section-body>p,.section-body>.bullet{background:#f8fafc;border:1px solid #edf1f7;border-radius:11px;padding:13px 16px;margin:0 0 12px}
.section-body>ul,.section-body>ol{margin:0 0 14px;padding-left:22px}
.section-body li{margin:6px 0}
.section-body>h3{margin:22px 0 12px}
.section-body>:last-child{margin-bottom:0}
.section-body figure{margin:20px 0}
.section-body table{margin:0 0 14px}

/* ---------- figures ---------- */
figure{background:#fff;border:1px solid var(--line);border-radius:14px;padding:18px;margin:22px 0;overflow:hidden}
figure svg{display:block;width:100%;height:auto;background:#fff}
figcaption{font-size:15px;color:#3c4f66;background:#f1f5fb;border-top:1px solid var(--line);padding:14px 18px;margin:18px -18px -18px}

/* ---------- practice questions ---------- */
.question-intro{background:#fff;border:1px solid var(--line);border-radius:14px;padding:18px 20px;margin:0 0 18px;font-size:15.5px;color:#3c4f66}
.question-intro p:last-child{margin:0}
#test article,.q-card{background:var(--paper);border:1px solid var(--line);border-radius:14px;box-shadow:var(--shadow);margin:0 0 14px;overflow:hidden}
#test article>h3,.q-card>h3{
  display:flex;align-items:center;gap:12px;margin:0;padding:12px 18px;font-size:15px;
  background:#16233a;color:#fff;font-weight:700;letter-spacing:.01em;
}
#test article>h3::before,.q-card>h3::before{
  content:attr(data-number);display:grid;place-items:center;min-width:28px;height:28px;
  background:var(--blue);border-radius:8px;font-size:15px;font-weight:800;
}
#test article>p,.q-card>p{margin:0;padding:18px}
#test details,.q-card details{border-top:1px solid var(--line);background:#f8fafc}
#test details summary,.q-card details summary{
  cursor:pointer;padding:13px 18px;font-weight:700;font-size:15px;color:#166534;list-style:none;
}
#test details summary::-webkit-details-marker{display:none}
#test details summary::before,.q-card details summary::before{content:"＋ ";font-weight:800}
#test details[open] summary::before,.q-card details[open] summary::before{content:"− "}
#test details summary:hover,.q-card details summary:hover{background:#eef7f0}
#test details>p,.q-card details>p{margin:0;padding:0 18px 18px;color:#2c3d52}
.answer-space{
  display:block;width:calc(100% - 36px);margin:0 18px 14px;padding:12px 14px;font:16px/1.9 inherit;
  color:var(--ink);background:repeating-linear-gradient(#fff,#fff 30px,#e8eef6 30px,#e8eef6 31px);
  border:1px solid var(--line);border-radius:10px;resize:vertical;
}
.answer-space::placeholder{color:#8c9bad;font-style:italic}
.answer-space:focus-visible{outline:3px solid #b45309;outline-offset:2px}

/* ---------- reading progress ---------- */
.read-progress{height:3px;background:#e2e8f0;position:sticky;top:66px;z-index:35}
.read-progress span{display:block;height:100%;width:0;background:var(--blue);transition:width .12s linear}
.toc-progress{margin:16px 0 0;font-size:13px;font-weight:700;color:var(--muted);display:flex;justify-content:space-between}
.toc-meter{height:6px;border-radius:999px;background:#e2e8f0;margin-top:7px;overflow:hidden}
.toc-meter span{display:block;height:100%;width:0;background:var(--navy);border-radius:999px;transition:width .12s linear}
.toc li a[aria-current=true]{color:var(--blue);border-left-color:var(--blue);background:#f1f5fd}

/* ---------- pager ---------- */
.lesson-pagination{display:flex;gap:14px;flex-wrap:wrap;margin:34px 0 0;padding-top:26px;border-top:1px solid var(--line)}
.lesson-pagination a{
  flex:1;min-width:210px;background:#fff;border:1px solid var(--line);border-radius:14px;
  padding:16px 18px;text-decoration:none;color:var(--ink);font-weight:700;box-shadow:var(--shadow);
}
.lesson-pagination a:hover{border-color:#9cb8e8;color:var(--blue)}
.lesson-pagination a[rel=next]{text-align:right}
.lesson-pagination .pager-index{text-align:center;color:var(--muted)}
.lesson-pagination .pager-index:hover{color:var(--blue)}

/* ---------- directory cards ---------- */
.grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(250px,1fr));gap:18px}
.grade-grid{grid-template-columns:repeat(auto-fit,minmax(190px,1fr))}
.directory-card{
  display:block;text-decoration:none;background:#fff;border:1px solid var(--line);border-radius:var(--radius);
  padding:22px;box-shadow:var(--shadow);transition:transform .14s,border-color .14s,box-shadow .14s;
}
.directory-card:hover{transform:translateY(-2px);border-color:#9cb8e8;box-shadow:0 10px 26px #0f172a12}
.directory-card h2{margin:0 0 6px;color:var(--ink);font-size:22px}
.directory-card p{color:var(--muted);font-size:15px;margin:0 0 14px}
.directory-card span{font-size:15px;font-weight:700;color:var(--blue)}
.directory-card .card-icon{
  display:grid;place-items:center;width:42px;height:42px;border-radius:12px;margin-bottom:14px;
  background:#eef2fb;color:var(--navy);font-size:17px;font-weight:800;
}

/* ---------- subject tabs ---------- */
.subject-tabs{display:flex;gap:8px;flex-wrap:wrap;margin:0 0 26px}
.subject-tabs a{
  padding:9px 16px;border:1px solid var(--line);border-radius:999px;text-decoration:none;
  background:#fff;color:#33465e;font-weight:600;font-size:15px;
}
.subject-tabs a:hover{border-color:#9cb8e8;color:var(--blue)}
.subject-tabs [aria-current=page]{background:var(--navy);color:#fff;border-color:var(--navy)}

/* ---------- lesson list ---------- */
.unit-heading{margin:30px 0 14px;font-size:15px;text-transform:uppercase;letter-spacing:.1em;color:var(--muted)}
.lesson-list{list-style:none;padding:0;margin:0}
.lesson-list li{margin:0 0 12px}
.lesson-list a{
  display:flex;align-items:center;gap:18px;text-decoration:none;background:#fff;
  border:1px solid var(--line);border-radius:14px;padding:18px 20px;box-shadow:var(--shadow);
  transition:transform .14s,border-color .14s;
}
.lesson-list a:hover{transform:translateY(-2px);border-color:#9cb8e8}
.lesson-list a>span:nth-child(2){flex:1}
.lesson-number{
  display:grid;place-items:center;min-width:38px;height:38px;border-radius:11px;
  background:#eef2fb;color:var(--navy);font-weight:800;font-size:15px;
}
.lesson-list strong{display:block;font-size:18px;color:var(--ink);letter-spacing:-.01em}
.lesson-list small{display:block;font-size:14px;color:var(--muted);margin-top:5px}
.lesson-list a>span:last-child{color:var(--blue);font-weight:800}

/* ---------- notices, sections, tables ---------- */
.notice{background:#fff;border:1px solid var(--line);border-left:5px solid #d97706;border-radius:14px;padding:18px 20px;margin:26px 0;box-shadow:var(--shadow)}
.notice strong{display:block;margin-bottom:6px}
.notice p{margin:0 0 10px;color:#3c4f66;font-size:15.5px}
.notice a{font-weight:700}
.reviewed-lessons,.framework-lessons{margin:0 0 26px}
.reviewed-lessons ul{list-style:none;padding:0;margin:0 0 12px;display:grid;gap:10px}
.reviewed-lessons li a{display:block;background:#fff;border:1px solid var(--line);border-radius:12px;padding:14px 18px;text-decoration:none;font-weight:700;color:var(--ink);box-shadow:var(--shadow)}
.reviewed-lessons li a:hover{border-color:#9cb8e8;color:var(--blue)}
.eyebrow{font-size:12.5px;font-weight:800;letter-spacing:.13em;text-transform:uppercase;color:var(--muted);margin:0 0 8px}
.lead{font-size:18px;color:#3c4f66;max-width:72ch}
.meta{font-size:15px;color:var(--muted)}
details.disclosure{background:#fff;border:1px solid var(--line);border-radius:14px;padding:18px 20px;margin:20px 0;box-shadow:var(--shadow)}
details.disclosure>summary{cursor:pointer;font-weight:700}
details.disclosure[open]>summary{margin-bottom:12px}
aside.sources{background:#fff;border:1px solid var(--line);border-radius:14px;padding:20px;margin:22px 0;box-shadow:var(--shadow)}
aside.sources h2{font-size:19px}
aside.sources ul{margin:0;padding-left:20px}
.scroll{overflow-x:auto;-webkit-overflow-scrolling:touch}
table{border-collapse:collapse;width:100%;font-size:15px;background:#fff}
th,td{border:1px solid var(--line);padding:10px 12px;text-align:left;vertical-align:top}
th{background:#f1f5fb;font-size:14px}

/* ---------- footer ---------- */
.site-footer{background:#0d1b30;color:#a9bdd8;margin-top:auto;border-top:4px solid var(--navy)}
.site-footer .shell{padding-top:40px;padding-bottom:32px}
.footer-cols{display:grid;grid-template-columns:repeat(auto-fit,minmax(190px,1fr));gap:28px}
.site-footer h2{font-size:12.5px;letter-spacing:.13em;text-transform:uppercase;color:#e2e9f5;margin-bottom:12px}
.site-footer ul{list-style:none;margin:0;padding:0;display:grid;gap:9px}
.site-footer .footer-grades ul{grid-template-columns:repeat(auto-fit,minmax(88px,1fr))}
.site-footer a{color:#c3d4ea;text-decoration:none;font-size:15px}
.site-footer a:hover{color:#fff;text-decoration:underline}
.site-footer .footer-note{border-top:1px solid #1f3050;margin-top:28px;padding-top:20px;font-size:13.5px;color:#8fa5c2;line-height:1.65}
body{display:flex;flex-direction:column;min-height:100vh}

@media(max-width:1000px){
  .lesson-layout{grid-template-columns:minmax(0,1fr);gap:0}
  .toc{position:static;margin-bottom:22px;background:#fff;border:1px solid var(--line);border-radius:14px;padding:16px 18px;box-shadow:var(--shadow)}
  .toc ol{display:flex;flex-wrap:wrap;gap:6px;border-left:0}
  .toc li a{border:1px solid var(--line);border-radius:999px;padding:7px 14px;margin:0;font-size:14px}
  .toc li a:hover{border-left-color:var(--line);background:#eef2fb}
}
@media(max-width:720px){
  body{font-size:16px}
  .shell{padding-left:16px;padding-right:16px}
  [id]{scroll-margin-top:16px}
  .site-header{position:static}
  .read-progress{position:static;top:auto}
  .site-header .shell{min-height:0;gap:10px;row-gap:8px}
  .brand{font-size:17px}
  .brand small{display:none}
  .draft-flag{margin-left:auto;font-size:11.5px;padding:4px 10px}
  .nav-pills{order:1;margin-left:0;width:100%;overflow-x:auto;flex-wrap:nowrap}
  .nav-pills a{padding:7px 13px;font-size:14px}
  .hero .shell{padding-top:26px;padding-bottom:26px}
  .section-body,.section-head{padding-left:16px;padding-right:16px}
  .lesson-list a{padding:15px;gap:12px}
  .lesson-list strong{font-size:16.5px}
  figure{padding:14px}
  figcaption{margin:14px -14px -14px;padding:13px 15px}
  .crumb-meta{margin-left:0}
  /* The lesson title is the last crumb; the h1 repeats it right below. */
  .breadcrumbs [aria-current=page]{max-width:26ch;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}
}
@media print{
  .site-header,.nav-pills,.toc,.lesson-pagination,.site-footer{display:none}
  body{background:#fff}
  .section-card,.directory-card,figure{box-shadow:none}
}
@media(prefers-reduced-motion:reduce){*{transition:none!important;animation:none!important}}