/* ============================================================
   BuyAI · base — reset / typography / focus / motion
   ============================================================ */

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

html {
  -webkit-text-size-adjust: 100%;
  scroll-behavior: smooth;
  scroll-padding-top: 76px;
}

body {
  margin: 0;
  background: var(--color-background);
  color: var(--color-foreground);
  font-family: var(--font-sans);
  font-size: var(--step-0);
  line-height: 1.68;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
  overflow-x: hidden;
  transition: background-color var(--dur-base) var(--ease-out), color var(--dur-base) var(--ease-out);
}

h1, h2, h3, h4 {
  margin: 0;
  font-family: var(--font-display);
  font-weight: 700;
  line-height: 1.08;
  letter-spacing: -.025em;
  text-wrap: balance;
}

h1 { font-size: var(--step-6); letter-spacing: -.035em; }
h2 { font-size: var(--step-4); letter-spacing: -.03em; }
h3 { font-size: var(--step-2); letter-spacing: -.02em; }
h4 { font-size: var(--step-1); font-weight: 600; letter-spacing: -.01em; }

p { margin: 0; }
ul, ol { margin: 0; padding: 0; list-style: none; }
img, svg { display: block; max-width: 100%; }

a { color: inherit; text-decoration: none; }

code, pre, kbd, samp {
  font-family: var(--font-mono);
  font-variant-numeric: tabular-nums;
}

button {
  font: inherit;
  color: inherit;
  background: none;
  border: 0;
  cursor: pointer;
}

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

.skip-link {
  position: absolute;
  left: 50%;
  top: 0;
  translate: -50% -120%;
  z-index: 99;
  padding: 10px 18px;
  background: var(--color-foreground);
  color: var(--color-background);
  font-size: var(--step--1);
  border-radius: 0 0 var(--radius-sm) var(--radius-sm);
  transition: translate var(--dur-fast) var(--ease-out);
}
.skip-link:focus { translate: -50% 0; }

.visually-hidden {
  position: absolute;
  width: 1px; height: 1px;
  margin: -1px; padding: 0;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

.container {
  width: 100%;
  max-width: var(--container);
  margin-inline: auto;
  padding-inline: var(--gutter);
}

.mono { font-family: var(--font-mono); font-variant-numeric: tabular-nums; }
.tnum { font-variant-numeric: tabular-nums; }

.lede {
  font-size: var(--step-1);
  line-height: 1.6;
  color: var(--color-foreground-muted);
  max-width: 60ch;
}

/* 章節標題組：標題 + 說明，間距靠這裡統一 */
.sec-head { display: grid; gap: var(--space-3); margin-bottom: var(--space-6); }
.sec-title { max-width: 30ch; }
.sec-sub { color: var(--color-foreground-subtle); max-width: 62ch; font-size: var(--step-0); }

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after {
    animation-duration: .001ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: .001ms !important;
  }
}