@font-face {
  font-family: "Open Sans";
  font-style: normal;
  font-weight: 300 800;
  font-display: swap;
  src: url("../assets/fonts/open-sans-latin.woff2") format("woff2");
}

@font-face {
  font-family: "Oswald";
  font-style: normal;
  font-weight: 200 700;
  font-display: swap;
  src: url("../assets/fonts/oswald-latin.woff2") format("woff2");
}


:root {
  --ink: #212121;
  --dark-ink: #1f1f1f;
  --accent: #fdd835;
}

*, *::before, *::after { box-sizing: border-box; }
[hidden] { display: none !important; }

body {
  margin: 0;
  color: var(--ink);
  font-family: "Open Sans", Arial, sans-serif;
  font-size: 16px;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}

a { color: inherit; text-underline-offset: 3px; }
a:hover { color: #17563d; }
a:focus-visible, button:focus-visible {
  outline: 3px solid #315f9f;
  outline-offset: 4px;
}
button, input { font: inherit; }
button { color: inherit; }

.skip-link {
  position: absolute;
  top: 8px;
  left: 12px;
  z-index: 10;
  padding: 10px 16px;
  background: #fff;
  transform: translateY(-160%);
}
.skip-link:focus { transform: translateY(0); }

.course-header {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 12px 24px;
  width: min(100%, 1080px);
  margin: 0 auto;
  padding: 20px 24px;
}
.course-author { font-weight: 650; text-decoration: none; }
.course-header nav { display: flex; flex-wrap: wrap; gap: 24px; }
.course-heading { margin-bottom: 28px; text-align: center; }
.course-heading p {
  margin: 0 0 8px;
  color: #4a514d;
  font-size: 14px;
}
.course-heading h1 {
  margin: 0;
  font-family: "Oswald", Arial, sans-serif;
  font-size: clamp(42px, 6vw, 58px);
  font-weight: 500;
  line-height: 1.2;
}

/* Practice screens: shared with the original exercise page. */
.practice-section {
  --practice-green: #237a57;
  --practice-green-dark: #17563d;
  --practice-mint: #e7f5ee;
  --practice-coral: #c43f4f;
  --practice-coral-soft: #fff0f2;
  --practice-blue: #315f9f;
  --practice-border: #d8dedb;
  padding: 52px 24px 72px;
  background:
    radial-gradient(circle at 8% 8%, rgba(253, 216, 53, 0.24), transparent 24rem),
    linear-gradient(180deg, #f8faf8 0%, #eef3f0 100%);
}

.practice-layout {
  width: min(100%, 940px);
  margin: 0 auto;
}

.practice-app {
  overflow: hidden;
  border: 1px solid var(--practice-border);
  border-top: 10px solid var(--accent);
  border-radius: 18px;
  background: #fff;
  box-shadow: 0 18px 48px rgba(25, 45, 34, 0.12);
}

.practice-screen {
  min-height: 520px;
  padding: 52px 64px;
}

.practice-screen[hidden] {
  display: none;
}

.practice-welcome,
.practice-results {
  text-align: center;
}

.practice-kicker,
.exercise-type,
.practice-counter,
.practice-topic {
  color: var(--practice-green);
  font-size: 13px;
  font-weight: 750;
  letter-spacing: 0.09em;
  text-transform: uppercase;
}

.practice-screen h2 {
  margin: 14px 0 0;
  color: var(--dark-ink);
  font-family: "Oswald", Arial, sans-serif;
  font-size: 42px;
  font-weight: 450;
  line-height: 1.16;
}

.practice-lead {
  max-width: 690px;
  margin: 20px auto 0;
  color: #4a514d;
  font-size: 18px;
  line-height: 1.7;
}

.practice-bank-picker {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px;
  margin: 28px 0 0;
  text-align: left;
}

.practice-bank {
  display: flex;
  flex-direction: column;
  gap: 5px;
  padding: 18px 20px;
  border: 2px solid var(--practice-border);
  border-radius: 12px;
  background: #fff;
  text-align: left;
  cursor: pointer;
}

.practice-bank strong { color: var(--dark-ink); font-size: 19px; }
.practice-bank span { color: #4a514d; font-size: 13px; line-height: 1.5; }
.practice-bank .practice-bank-range { color: var(--practice-green-dark); font-weight: 700; }
.practice-bank:hover { border-color: #9db5a7; }
.practice-bank.is-selected { border-color: var(--practice-green); background: var(--practice-mint); }
.practice-bank:disabled { opacity: 0.6; cursor: not-allowed; }
.exercise-feedback .exercise-source { margin-top: 12px; font-size: 14px; }

.exercise-context {
  display: grid;
  gap: 16px;
  padding: 18px 20px;
  margin: 18px 0 20px;
  border: 1px solid var(--practice-border);
  border-radius: 10px;
  background: #f7faf8;
  text-align: left;
  overflow-wrap: anywhere;
}
.exercise-context > div { min-width: 0; }
.exercise-context strong { color: var(--practice-green-dark); font-size: 14px; }
.exercise-context p { margin: 5px 0 0; font-size: 16px; line-height: 1.6; white-space: pre-line; }

/* Explicit TeX is rendered locally. Long expressions scroll within their block,
   never widening a mobile question or shrinking mathematics to unreadable size. */
.exercise-math {
  display: inline-block;
  max-width: 100%;
  vertical-align: middle;
  overflow-x: auto;
  overflow-y: hidden;
  line-height: normal;
  font-weight: normal;
  white-space: normal;
  padding: 0.12em 0;
}
.exercise-math[data-display="true"] {
  display: block;
  text-align: center;
  margin: 0.6em 0;
  padding: 0.3em 0;
}
.exercise-math mjx-container { margin: 0 !important; }
/* Preserve screen-reader MathML without a second visible copy of the formula. */
.exercise-math mjx-assistive-mml {
  position: absolute !important;
  top: 0;
  left: 0;
  width: 1px;
  height: 1px;
  padding: 0 !important;
  border: 0 !important;
  overflow: hidden !important;
  clip: rect(1px, 1px, 1px, 1px);
  white-space: nowrap !important;
}
.exercise-context p, .exercise-feedback p { line-height: 1.75; }
.exercise-option-text { min-width: 0; }

/* Data-driven, local diagrams. The question always also states the data in text. */
.exercise-visual {
  margin: 22px 0;
  padding: 16px;
  border: 1px solid #d6e6e1;
  border-radius: 16px;
  background: #f7fbfa;
}
.exercise-visual > svg { display: block; width: 100%; max-width: 640px; height: auto; margin: 0 auto; }
.exercise-visual text { font-family: inherit; font-size: 18px; fill: #24473e; }
.exercise-visual .visual-axis { stroke: #647a74; stroke-width: 1.5; }
.exercise-visual .visual-guide { stroke: #c9dbd5; stroke-width: 1; stroke-dasharray: 5 5; }
.exercise-visual .visual-fill { fill: #007761; }
.exercise-visual .visual-soft { fill: #c6e8db; }
.exercise-visual .visual-curve { fill: none; stroke: #007761; stroke-width: 3; }
.exercise-visual figcaption, .exercise-visual .visual-key { margin: 8px 0 0; font-size: 14px; line-height: 1.5; color: #365b51; }
.exercise-visual figcaption strong { display: block; margin-bottom: 3px; }

.practice-stats {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 12px;
  max-width: 620px;
  margin: 32px auto 0;
}

.practice-stat,
.results-grid > div {
  display: flex;
  flex-direction: column;
  justify-content: center;
  min-height: 92px;
  padding: 14px;
  border: 1px solid var(--practice-border);
  border-radius: 12px;
  background: #fbfcfb;
}

.practice-stat strong,
.results-grid strong {
  color: var(--practice-green-dark);
  font-family: "Oswald", Arial, sans-serif;
  font-size: 30px;
  font-weight: 500;
  line-height: 1;
}

.practice-stat span,
.results-grid span {
  margin-top: 8px;
  color: #616963;
  font-size: 13px;
  letter-spacing: 0.04em;
  text-transform: uppercase;
}

.practice-topics {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 8px;
  margin: 28px auto;
}

.practice-topics span {
  padding: 7px 11px;
  border-radius: 999px;
  background: var(--practice-mint);
  color: var(--practice-green-dark);
  font-size: 13px;
  font-weight: 650;
}

.practice-button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 48px;
  padding: 11px 22px;
  border: 2px solid transparent;
  border-radius: 10px;
  font-weight: 700;
  line-height: 1.35;
  text-align: center;
  text-decoration: none;
  cursor: pointer;
  transition: transform 100ms ease, background-color 100ms ease, box-shadow 100ms ease;
}

.practice-button[hidden],
.exercise-feedback[hidden] {
  display: none;
}

.practice-button:hover:not(:disabled) {
  transform: translateY(-1px);
}

.practice-button:focus-visible,
.exercise-option:focus-within,
.exercise-number:focus-visible {
  outline: 3px solid rgba(49, 95, 159, 0.3);
  outline-offset: 3px;
}

.practice-button-primary {
  background: var(--practice-green);
  box-shadow: 0 4px 0 var(--practice-green-dark);
  color: #fff;
}

.practice-button-primary:hover:not(:disabled) {
  background: #1d6d4d;
}

.practice-button-primary:disabled {
  background: #bac4be;
  box-shadow: 0 4px 0 #929e97;
  cursor: not-allowed;
}

.practice-button-quiet {
  border-color: var(--practice-border);
  background: #fff;
  color: #4b544f;
}

.practice-button-quiet:hover {
  background: #f4f7f5;
}

.practice-note,
.practice-source-note {
  color: #69716c;
  font-size: 13px;
  line-height: 1.55;
}

.practice-note {
  margin: 20px 0 0;
}

.practice-session-bar,
.practice-actions {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 18px;
}

.practice-counter,
.practice-topic {
  display: block;
}

.practice-topic {
  margin-top: 5px;
  color: #747d77;
  font-size: 11px;
}

.practice-scoreboard {
  display: flex;
  gap: 16px;
  color: #606963;
  font-size: 14px;
}

.practice-scoreboard strong {
  color: var(--practice-green-dark);
}

.practice-progress {
  height: 10px;
  margin-top: 20px;
  overflow: hidden;
  border-radius: 999px;
  background: #e5eae7;
}

.practice-progress span {
  display: block;
  width: 0;
  height: 100%;
  border-radius: inherit;
  background: linear-gradient(90deg, var(--accent), #f4bd2d);
  transition: width 220ms ease;
}

.exercise-card {
  min-height: 315px;
  padding: 34px 0 20px;
}

.exercise-card h2 {
  max-width: 760px;
  margin-top: 10px;
  font-family: "Open Sans", Arial, sans-serif;
  font-size: 27px;
  font-weight: 700;
  line-height: 1.35;
}

.exercise-instruction {
  margin: 12px 0 0;
  color: #667069;
  font-size: 15px;
  line-height: 1.55;
}

.exercise-answer-area {
  display: grid;
  gap: 10px;
  margin-top: 24px;
}

.exercise-option {
  position: relative;
  display: grid;
  grid-template-columns: 32px 1fr;
  align-items: center;
  gap: 12px;
  min-height: 58px;
  padding: 10px 16px;
  border: 2px solid #dce2de;
  border-radius: 11px;
  background: #fff;
  color: #333a36;
  cursor: pointer;
  transition: border-color 100ms ease, background-color 100ms ease, transform 100ms ease;
}

.exercise-option:hover {
  border-color: #aebbb3;
  transform: translateY(-1px);
}

.exercise-option.is-selected {
  border-color: var(--practice-blue);
  background: #f0f5fc;
}

.exercise-option.is-correct {
  border-color: var(--practice-green);
  background: var(--practice-mint);
}

.exercise-option.is-incorrect {
  border-color: var(--practice-coral);
  background: var(--practice-coral-soft);
}

.exercise-option input {
  position: absolute;
  opacity: 0;
  pointer-events: none;
}

.exercise-option-marker {
  display: grid;
  width: 30px;
  height: 30px;
  border: 1px solid #bbc5bf;
  border-radius: 8px;
  background: #f7f9f8;
  color: #59635d;
  font-size: 13px;
  font-weight: 750;
  place-items: center;
}

.exercise-option-text {
  font-size: 16px;
  line-height: 1.45;
}

.exercise-number-wrap {
  display: flex;
  align-items: center;
  max-width: 390px;
  border: 2px solid #cfd8d2;
  border-radius: 11px;
  background: #fff;
}

.exercise-number-wrap:focus-within {
  border-color: var(--practice-blue);
}

.exercise-number {
  min-width: 0;
  width: 100%;
  height: 58px;
  padding: 0 16px;
  border: 0;
  border-radius: inherit;
  outline: 0;
  color: var(--dark-ink);
  font-size: 20px;
}

.exercise-number-unit {
  padding-right: 16px;
  color: #68716c;
  white-space: nowrap;
}

.exercise-feedback {
  padding: 18px 20px;
  margin: 4px 0 22px;
  border-left: 6px solid var(--practice-green);
  border-radius: 8px;
  background: var(--practice-mint);
  color: #274437;
}

.exercise-feedback.is-incorrect {
  border-left-color: var(--practice-coral);
  background: var(--practice-coral-soft);
  color: #672b33;
}

.exercise-feedback strong {
  font-size: 17px;
}

.exercise-feedback p {
  margin: 6px 0 0;
  line-height: 1.55;
}

.practice-actions {
  padding-top: 20px;
  border-top: 1px solid #e3e7e5;
}

.practice-actions-centered {
  justify-content: center;
  margin-top: 30px;
}

.results-emblem {
  display: grid;
  width: 72px;
  height: 72px;
  margin: 0 auto 18px;
  border-radius: 50%;
  background: var(--practice-mint);
  color: var(--practice-green);
  font-size: 34px;
  font-weight: 800;
  place-items: center;
}

.results-score {
  display: flex;
  flex-direction: column;
  margin: 28px 0 0;
  color: #626b65;
}

.results-score strong {
  color: var(--practice-green-dark);
  font-family: "Oswald", Arial, sans-serif;
  font-size: 70px;
  font-weight: 500;
  line-height: 1;
}

.results-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 12px;
  max-width: 620px;
  margin: 28px auto 0;
}

.practice-source-note {
  padding: 16px 20px;
  margin: 18px auto 0;
  border-left: 4px solid var(--accent);
  background: rgba(255, 255, 255, 0.7);
}

.practice-noscript {
  padding: 16px;
  margin: 20px;
  border: 1px solid var(--practice-coral);
  background: var(--practice-coral-soft);
}


/* Narrow screens and keyboard navigation. */
.exercise-option-text, .exercise-card h2, .exercise-feedback {
  min-width: 0;
  overflow-wrap: anywhere;
}
.exercise-number-wrap { width: 100%; }
.exercise-option:has(input[type="checkbox"]) .exercise-option-marker { border-radius: 4px; }
.exercise-option:has(input[type="checkbox"]:checked) .exercise-option-marker::after { content: "✓"; }
.visual-series-key { display: flex; flex-wrap: wrap; gap: 10px 20px; }
.practice-actions { flex-wrap: wrap; }
.exercise-number-wrap:focus-within {
  outline: 3px solid rgba(49, 95, 159, 0.3);
  outline-offset: 3px;
}
.exercise-number:focus-visible { outline: none; }

@media (max-width: 767px) {
  .exercise-visual text { font-size: 24px; }
  .course-header { padding: 16px 20px; }
  .practice-section { padding: 26px 14px 40px; }
  .practice-screen { min-height: 0; padding: 30px 24px; }
  .practice-screen h2 { font-size: 32px; }
  .practice-lead { font-size: 16px; }
  .practice-bank-picker { grid-template-columns: 1fr; }
  .practice-session-bar, .practice-actions {
    align-items: stretch;
    flex-direction: column;
  }
  .practice-session-bar { align-items: flex-start; }
  .practice-scoreboard { width: 100%; justify-content: space-between; }
  .practice-actions .practice-button { width: 100%; }
  .exercise-card { min-height: 0; padding-top: 26px; }
  .exercise-card h2 { font-size: 23px; }
}

@media (max-width: 479px) {
  .course-header { font-size: 14px; }
  .course-header nav { gap: 20px; }
  .course-heading p { font-size: 12px; }
  .practice-app { border-radius: 12px; }
  .practice-screen { padding: 26px 18px; }
  .practice-bank { padding: 14px 16px; }
  .practice-stat, .results-grid > div { min-height: 78px; padding: 10px 6px; }
  .practice-stat span, .results-grid span { font-size: 10px; letter-spacing: 0; }
  .practice-stats, .results-grid { gap: 8px; }
  .exercise-option { grid-template-columns: 28px minmax(0, 1fr); padding: 10px 12px; }
  .exercise-option-marker { width: 27px; height: 27px; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    scroll-behavior: auto !important;
    transition-duration: 0.01ms !important;
  }
}