/* ============================================================
   LAYERS — cascade order declared up front, per the product's
   own "we teach modern CSS by using it" thesis.
   ============================================================ */
@layer reset, tokens, base, components, utilities;

@layer reset {
  *, *::before, *::after { box-sizing: border-box; }
  html, body { height: 100%; }
  body { margin: 0; }
  h1, h2, h3, p, dl, dd, figure { margin: 0; }
  ul, ol { margin: 0; padding: 0; list-style: none; }
  button, textarea, input, select { font: inherit; color: inherit; }
  button { background: none; border: none; cursor: pointer; }
  img, svg { display: block; max-width: 100%; }
  a { color: inherit; }
}

@layer tokens {
  :root {
    color-scheme: light;

    /* ---- accent (single hue: green) ---- */
    --accent-h: 148;
    --accent: oklch(0.5 0.16 var(--accent-h));
    --accent-strong: oklch(0.42 0.18 var(--accent-h));
    --accent-dim: oklch(0.68 0.09 var(--accent-h));

    /* ---- neutrals (true grey/black/white, faint green lean) ---- */
    --bg: oklch(0.98 0.003 148);
    --bg-raised: oklch(1 0 0);
    --bg-inset: oklch(0.95 0.005 148);
    --border: oklch(0.85 0.008 148);
    --border-soft: oklch(0.9 0.005 148);
    --text: oklch(0.22 0.008 148);
    --text-dim: oklch(0.4 0.01 148);
    --text-faint: oklch(0.56 0.01 148);

    /* ---- semantic (ok = brighter green than chrome accent, so "solved" still pops) ---- */
    --ok: oklch(0.56 0.17 138);
    --ok-bg: oklch(0.92 0.05 138);
    --bad: oklch(0.5 0.14 22);
    --bad-bg: oklch(0.94 0.05 22);

    --radius: 12px;
    --radius-sm: 8px;
    --font-mono: ui-monospace, "Cascadia Code", "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;
    --font-ui: system-ui, -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    --ease: cubic-bezier(.2,.7,.2,1);
    --dur: 200ms;
  }

  /* animatable custom property backing the progress rail fill */
  @property --fill {
    syntax: '<percentage>';
    inherits: true;
    initial-value: 0%;
  }
}

@layer base {
  html { -webkit-text-size-adjust: 100%; overflow-x: hidden; }
  body {
    background: var(--bg);
    color: var(--text);
    font-family: var(--font-ui);
    line-height: 1.5;
    overflow-x: hidden;
    max-inline-size: 100vw;
  }
  code, pre, kbd, textarea.editor { font-family: var(--font-mono); }
  :focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
    border-radius: 4px;
  }
  ::selection { background: color-mix(in oklch, var(--accent) 35%, transparent); }
}

@layer components {

  .skip-link {
    position: absolute;
    inset-block-start: -100%;
    inset-inline-start: 1rem;
    background: var(--accent);
    color: black;
    padding: .5rem 1rem;
    border-radius: var(--radius-sm);
    z-index: 100;
  }
  .skip-link:focus { inset-block-start: .5rem; }

  /* ---------- sticky progress rail ---------- */
  .rail {
    position: sticky;
    inset-block-start: 0;
    z-index: 20;
    display: flex;
    align-items: center;
    gap: 1rem;
    padding-inline: clamp(1rem, 4vw, 2.5rem);
    padding-block: .85rem;
    background: #000;
    border-block-end: 1px solid var(--border-soft);
    /* Rail stays a dark chip regardless of the page's light theme. */
    --text: #fff;
    --text-dim: #cfcfcf;
    --text-faint: #8a8a8a;
    --border: #444;
    --border-soft: #2a2a2a;
    --bg-inset: #1a1a1a;
    color: var(--text);
  }
  .rail__brand {
    font-weight: 600;
    letter-spacing: -.01em;
    white-space: nowrap;
    font-size: .95rem;
  }
  .rail__brand em {
    font-style: normal;
    color: var(--accent);
  }
  .rail__track {
    flex: 1;
    height: 6px;
    border-radius: 999px;
    background: var(--bg-inset);
    overflow: hidden;
    min-inline-size: 60px;
  }
  .rail__fill {
    --fill: 0%;
    height: 100%;
    inline-size: var(--fill);
    background: linear-gradient(90deg, var(--accent-dim), var(--accent-strong));
    border-radius: inherit;
    transition: --fill var(--dur) var(--ease);
  }
  .rail__count {
    font-family: var(--font-mono);
    font-size: .85rem;
    color: var(--text-dim);
    white-space: nowrap;
  }
  .rail__dots { display: flex; gap: .4rem; }
  .rail__dot {
    inline-size: 9px; block-size: 9px; border-radius: 50%;
    background: var(--border);
    transition: background var(--dur) var(--ease), transform var(--dur) var(--ease);
  }
  .rail__dot[data-solved="true"] { background: var(--accent); transform: scale(1.15); }

  /* ---------- page shell ---------- */
  .hero-copy {
    padding: clamp(1.5rem, 5vw, 3.5rem) clamp(1rem, 4vw, 2.5rem) 1rem;
  }
  .hero-copy h1 {
    font-size: clamp(1.8rem, 4vw, 2.6rem);
    letter-spacing: -.02em;
  }
  .hero-copy p {
    color: var(--text-dim);
    margin-block-start: .35rem;
  }

  /* ---------- popovers (info / teach) ---------- */
  .info-btn {
    font-family: var(--font-mono);
    font-size: .72rem;
    color: var(--text-dim);
    border: 1px solid var(--border);
    border-radius: 999px;
    padding: .35rem .8rem;
    background: var(--bg-inset);
    white-space: nowrap;
    transition: border-color var(--dur) var(--ease), color var(--dur) var(--ease);
  }
  .info-btn:hover { border-color: var(--accent-dim); color: var(--text); }
  [popover].pop {
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
    background: var(--bg-raised);
    color: var(--text);
    padding: 1rem 1.1rem;
    max-inline-size: 300px;
    margin: auto;
    box-shadow: 0 12px 40px oklch(0 0 0 / .5);
  }
  [popover].pop::backdrop { background: oklch(0 0 0 / .4); }
  .pop__title {
    font-family: var(--font-mono); font-size: .78rem; font-weight: 600;
    color: var(--accent-strong); text-transform: uppercase; letter-spacing: .04em;
    margin-block-end: .6rem;
  }
  .pop__group + .pop__group { margin-block-start: .8rem; }
  .pop__group h4 { font-size: .82rem; color: var(--text); margin-block-end: .35rem; font-family: var(--font-mono); }
  .pop__list { display: grid; gap: .4rem; }
  .pop__list li { font-size: .84rem; line-height: 1.4; color: var(--text-dim); padding-inline-start: 1rem; position: relative; }
  .pop__list li::before { content: '•'; position: absolute; inset-inline-start: 0; color: var(--accent); }

  main { padding: 0 clamp(1rem, 4vw, 2.5rem) 4rem; display: grid; gap: 2.25rem; }

  /* ---------- exercise card ---------- */
  .exercise {
    container-type: inline-size;
    container-name: exercise;
    background: var(--bg-raised);
    border: 1px solid var(--border-soft);
    border-radius: var(--radius);
    overflow: hidden;
    transition: border-color var(--dur) var(--ease);
  }
  .exercise[data-solved="true"] { border-color: color-mix(in oklch, var(--accent) 50%, var(--border-soft)); }

  .exercise__head {
    display: flex; flex-wrap: wrap; align-items: baseline; gap: .6rem;
    padding: 1.1rem 1.3rem;
    border-block-end: 1px solid var(--border-soft);
  }
  .exercise__num {
    font-family: var(--font-mono); font-size: .8rem; color: var(--text-faint);
  }
  .exercise__title { font-size: 1.05rem; font-weight: 600; letter-spacing: -.01em; }
  .exercise__track-badge {
    font-family: var(--font-mono);
    font-size: .68rem;
    text-transform: uppercase;
    letter-spacing: .06em;
    padding: .18rem .5rem;
    border-radius: 999px;
    border: 1px solid var(--border);
    color: var(--text-dim);
  }
  .exercise__concept { color: var(--text-faint); font-size: .85rem; margin-inline-start: auto; order: 10; }
  .exercise__prompt {
    padding: .9rem 1.3rem 0;
    font-size: .88rem;
    color: var(--text-dim);
  }
  .exercise__prompt:empty { display: none; }
  .exercise__solved-pill {
    font-size: .72rem; font-family: var(--font-mono);
    color: var(--ok); background: color-mix(in oklch, var(--ok) 16%, transparent);
    padding: .2rem .55rem; border-radius: 999px;
    opacity: 0; transform: translateY(-2px);
    transition: opacity var(--dur) var(--ease), transform var(--dur) var(--ease);
  }
  .exercise[data-solved="true"] .exercise__solved-pill { opacity: 1; transform: none; }

  /* ---------- predict quiz ---------- */
  .predict {
    margin-block-start: .5rem;
    border: 1px dashed var(--border);
    border-radius: var(--radius-sm);
    padding: .8rem .9rem;
  }
  .predict p.q { font-size: .88rem; color: var(--text); margin-block-end: .6rem; }
  .predict__opts { display: grid; gap: .4rem; }
  .predict__opt {
    text-align: start;
    border: 1px solid var(--border-soft);
    border-radius: var(--radius-sm);
    padding: .5rem .7rem;
    font-size: .84rem;
    background: var(--bg-raised);
    transition: border-color var(--dur) var(--ease), background var(--dur) var(--ease);
  }
  .predict__opt:hover { border-color: var(--accent-dim); }
  .predict__opt[data-state="correct"] { border-color: var(--ok); background: color-mix(in oklch, var(--ok) 12%, var(--bg-raised)); }
  .predict__opt[data-state="wrong"] { border-color: var(--bad); background: color-mix(in oklch, var(--bad) 10%, var(--bg-raised)); }
  .predict__fb { font-size: .82rem; color: var(--text-dim); margin-block-start: .5rem; }

  /* ---------- work area: editor + preview ---------- */
  .panes {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 1px;
    background: var(--border-soft);
  }
  @container exercise (max-width: 840px) {
    .panes { grid-template-columns: 1fr; }
  }
  .pane { background: var(--bg-raised); display: flex; flex-direction: column; min-inline-size: 0; }
  .pane__label {
    font-family: var(--font-mono); font-size: .72rem; text-transform: uppercase; letter-spacing: .06em;
    color: var(--text-faint); padding: .6rem .9rem; border-block-end: 1px solid var(--border-soft);
    display: flex; justify-content: space-between; align-items: center;
  }
  .pane__label .w { color: var(--text-dim); }

  .editor-wrap {
    position: relative;
    flex: 1;
    display: flex;
    min-block-size: 220px;
    background: var(--bg-inset);
  }
  .editor-highlight,
  textarea.editor {
    position: absolute;
    inset: 0;
    margin: 0;
    padding: .9rem 1rem;
    font-size: .84rem;
    line-height: 1.6;
    tab-size: 2;
    -moz-tab-size: 2;
    white-space: pre-wrap;
    word-break: break-word;
    overflow: auto;
  }
  .editor-highlight {
    color: var(--text);
    pointer-events: none;
    z-index: 1;
  }
  textarea.editor {
    resize: vertical;
    background: transparent;
    color: transparent;
    caret-color: var(--text);
    border: none;
    z-index: 2;
  }
  textarea.editor::selection { background: color-mix(in oklch, var(--accent) 35%, transparent); }

  .tok-comment { color: var(--text-faint); font-style: italic; }
  .tok-string { color: var(--ok); }
  .tok-at-rule { color: var(--accent-strong); }
  .tok-property { color: var(--accent-strong); }
  .tok-pseudo { color: oklch(0.5 0.18 320); }
  .tok-selector-part { color: oklch(0.5 0.16 90); }
  .tok-number { color: oklch(0.52 0.18 60); }
  .tok-value { color: var(--text); }
  .tok-punctuation { color: var(--text-faint); }
  .preview-wrap { flex: 1; position: relative; min-block-size: 220px; background: repeating-linear-gradient(45deg, var(--bg-inset), var(--bg-inset) 10px, var(--bg-raised) 10px, var(--bg-raised) 20px); }
  .preview-wrap iframe.live {
    position: absolute; inset: 0; width: 100%; height: 100%; border: 0; background: var(--bg-raised);
  }

  /* ---------- controls / results ---------- */
  .controls {
    display: flex; flex-wrap: wrap; align-items: center; gap: .6rem;
    padding: .9rem 1.3rem;
    border-block-start: 1px solid var(--border-soft);
  }
  .btn {
    font-size: .85rem;
    padding: .55rem 1rem;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
    background: var(--bg-inset);
    color: var(--text);
    transition: border-color var(--dur) var(--ease), background var(--dur) var(--ease), transform var(--dur) var(--ease);
  }
  .btn:hover { border-color: var(--accent-dim); }
  .btn:active { transform: translateY(1px); }
  .btn--primary { background: var(--accent); color: #fff; border-color: transparent; font-weight: 600; }
  .btn--primary:hover { background: var(--accent-strong); }
  .btn--ghost { background: transparent; }
  .btn[disabled] { opacity: .45; cursor: not-allowed; }
  .hint-count { font-size: .78rem; color: var(--text-faint); font-family: var(--font-mono); }

  .results {
    padding: 0 1.3rem 1.1rem;
    display: grid; gap: .35rem;
  }
  .results:empty { display: none; }
  .result-group__title {
    font-family: var(--font-mono); font-size: .72rem; color: var(--text-faint);
    text-transform: uppercase; letter-spacing: .05em;
    margin-block: .6rem .3rem;
  }
  .result-line {
    display: flex; gap: .55rem; align-items: flex-start;
    font-size: .84rem; padding: .45rem .6rem; border-radius: var(--radius-sm);
    background: var(--bg-inset);
  }
  .result-line .mark { font-family: var(--font-mono); flex-shrink: 0; }
  .result-line[data-ok="true"] .mark { color: var(--ok); }
  .result-line[data-ok="false"] .mark { color: var(--bad); }
  .result-line[data-ok="false"] { background: color-mix(in oklch, var(--bad) 7%, var(--bg-inset)); }
  .result-line code { font-family: var(--font-mono); font-size: .82em; color: var(--accent-strong); }

  .hints { padding: 0 1.3rem 1.1rem; display: grid; gap: .5rem; }
  .hint-item {
    font-size: .84rem; color: var(--text-dim);
    padding: .6rem .8rem; border: 1px solid var(--border-soft); border-radius: var(--radius-sm);
    background: var(--bg-inset);
  }
  .hint-item b { color: var(--accent-strong); font-family: var(--font-mono); font-size: .75rem; }

  .solution-reveal {
    margin: 0 1.3rem 1.1rem;
    padding: .8rem .9rem;
    border: 1px solid var(--border-soft);
    border-radius: var(--radius-sm);
    background: var(--bg-inset);
  }
  .solution-reveal pre {
    font-size: .8rem; overflow-x: auto; color: var(--text-dim); margin-block-start: .5rem;
    white-space: pre-wrap; word-break: break-word;
  }

  .success {
    margin: 0 1.3rem 1.3rem;
    padding: 1rem 1.1rem;
    border-radius: var(--radius-sm);
    background: color-mix(in oklch, var(--ok) 14%, var(--bg-raised));
    border: 1px solid color-mix(in oklch, var(--ok) 40%, var(--border-soft));
    animation: rise var(--dur) var(--ease);
  }
  .success h3 { color: var(--ok); font-size: .95rem; margin-block-end: .5rem; }
  .success .pop__list { max-inline-size: 68ch; }
  @keyframes rise { from { opacity: 0; transform: translateY(4px); } to { opacity: 1; transform: none; } }
}

@layer utilities {
  .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;
  }
  .mono { font-family: var(--font-mono); }
  .hidden-frame {
    position: absolute;
    inset-inline-start: -99999px;
    inset-block-start: 0;
    visibility: hidden;
    border: 0;
    height: 900px;
    pointer-events: none;
  }
}

@media (prefers-reduced-motion: reduce) {
  * { animation-duration: 0.001ms !important; animation-iteration-count: 1 !important; transition-duration: 0.001ms !important; scroll-behavior: auto !important; }
}