/* base.css: design tokens, reset, typography. Light mode only. */

:root {
  color-scheme: light;

  /* surfaces and text */
  --bg: #fbfaf7;
  --surface: #ffffff;
  --fg: #1d1d1b;
  --muted: #6b6b66;
  --border: #dcdad3;
  --border-strong: #b8b5ac;

  /* accent (links, focus, active rail dot) */
  --accent: #2455c8;
  --accent-soft: #e4ebfa;

  /* the two curves the whole piece is about */
  --exact: #2455c8;     /* blue: closed-form solution */
  --approx: #d43a2f;    /* red: the integrator's polyline */

  /* verdicts */
  --stable: #1a9c4a;
  --unstable: #d43a2f;

  /* the four force models of Part I, one hue each (panels 2 and 5). Deliberately clear of
     --exact/--approx/--stable/--unstable, which mean something else everywhere in the piece,
     and far enough apart to tell four arrows apart on one plot. */
  --force-wind: #0f8a92;
  --force-gravity: #6b3fa0;
  --force-drag: #b3701a;
  --force-spring: #b5218f;

  /* the stability region (shader fill and edge) */
  --region: #4079d9;
  --region-edge: #0d3399;
  --region-none: #e9e8e3;  /* methods with no scalar R(hλ) */

  /* plot furniture */
  --grid: rgba(0, 0, 0, 0.08);
  --axis: #3a3a37;
  --tick: #6b6b66;

  /* type */
  --font: system-ui, -apple-system, "Segoe UI", Helvetica, Arial, sans-serif;
  --mono: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  /* the measure is a character count, not a length: 38 characters of whatever type size the
     block it applies to is set in. At the root that is 38rem; inside a `.prose` that fit.js has
     scaled down it follows the type, so a shortened column keeps its line length. */
  --measure-chars: 38;
  --measure: calc(var(--measure-chars) * 1rem);

  --radius: 6px;
  --shadow: 0 1px 2px rgba(0, 0, 0, 0.06), 0 8px 24px rgba(0, 0, 0, 0.06);
}

*, *::before, *::after { box-sizing: border-box; }

html {
  font: 16px/1.55 var(--font);
  color: var(--fg);
  background: var(--bg);
  -webkit-text-size-adjust: 100%;
}

body { margin: 0; }

h1, h2, h3, h4 {
  margin: 0 0 0.5em;
  line-height: 1.2;
  font-weight: 600;
  letter-spacing: -0.01em;
}
h1 { font-size: 2.25rem; }
h2 { font-size: 1.75rem; }
h3 { font-size: 1.2rem; }
h4 { font-size: 1rem; }

p, ul, ol { margin: 0 0 1em; max-width: var(--measure); }
li { margin-bottom: 0.25em; }

a { color: var(--accent); text-underline-offset: 0.15em; }
a:hover { text-decoration-thickness: 2px; }

strong { font-weight: 600; }
small, .muted { color: var(--muted); }

code, kbd, .mono {
  font-family: var(--mono);
  font-size: 0.9em;
  font-variant-numeric: tabular-nums;
}
code { background: var(--accent-soft); padding: 0.05em 0.3em; border-radius: 3px; }

/* Native MathML. No typesetting library. */
math { font-size: 1.05em; }
/* A live number (data-var/data-scrub) can grow a digit while a pane runs; MathML never
   wraps its own content onto a second line, so without this a formula that grows past its
   column just bleeds into whatever is beside it. Contained, not clipped: it scrolls. */
math[display="block"] { display: block; margin: 0.75em 0; max-width: 100%; overflow-x: auto; }

button { font: inherit; color: inherit; }

:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }

.skip {
  position: absolute; left: -999px; top: 0.5rem;
  background: var(--surface); padding: 0.5rem 0.75rem; border-radius: var(--radius); z-index: 100;
}
.skip:focus { left: 0.5rem; }

.noscript { padding: 2rem; }