/* ===========================================================================
   tau.education — reset, typography, layout primitives
   =========================================================================== */

*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--fg-body);
  font: 400 17px/1.7 var(--font-text);
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
  overflow-x: hidden;
}

img, svg { max-width: 100%; display: block; }
a { color: var(--accent); text-decoration: none; transition: color var(--dur) var(--ease); }
a:hover { color: var(--accent-hover); }

::selection { background: var(--accent); color: #0F0F0F; }

/* Keyboard focus — obligatory (IMPLEMENTATION §4) */
:focus-visible { outline: 1px solid var(--accent); outline-offset: 2px; }
:focus:not(:focus-visible) { outline: none; }

p { text-wrap: pretty; }

/* --- Headings: weight is always 400; hierarchy is size + colour --- */
h1, h2, h3 { font-weight: 400; margin: 0; color: var(--fg-display); letter-spacing: -.01em; }
h3 { color: var(--fg); letter-spacing: 0; }

.h1-hero      { font: 400 clamp(42px, 7.4vw, 102px)/1.0 var(--font-text); letter-spacing: -.025em; }
@media (max-width: 620px) { .h1-hero br { display: none; } }
.h1-page      { font: 400 clamp(31px, 4.6vw, 56px)/1.1 var(--font-text); letter-spacing: -.018em; }
.h2-xl        { font: 400 clamp(30px, 4vw, 60px)/1.08 var(--font-text); letter-spacing: -.02em; }
.h2-lg        { font: 400 clamp(28px, 3.4vw, 46px)/1.17 var(--font-text); }
.h2-md        { font: 400 clamp(26px, 3vw, 40px)/1.18 var(--font-text); }
.h3-row       { font: 400 clamp(21px, 2.2vw, 27px)/1.2 var(--font-text); }
.h3-card      { font: 400 clamp(19px, 1.6vw, 22px)/1.3 var(--font-text); }
.h3-sm        { font: 400 18px/1.3 var(--font-text); }

.lead    { font: 400 clamp(16px, 1.5vw, 18.5px)/1.65 var(--font-text); color: var(--fg-body); max-width: 40em; }
.body    { font: 400 clamp(15.5px, 1.3vw, 17px)/1.7 var(--font-text); color: var(--fg-body); }
.body-sm { font: 400 15.5px/1.65 var(--font-text); color: var(--fg-muted); }

.eyebrow {
  font: 500 10.5px/1 var(--font-mono);
  letter-spacing: .22em; text-transform: uppercase;
  color: var(--accent);
}
.label-mono {
  font: 500 10px/1 var(--font-mono);
  letter-spacing: .16em; text-transform: uppercase;
  color: var(--fg-meta);
}
.meta-mono { font: 400 13px/1.6 var(--font-mono); color: var(--fg-faint); }

/* --- Layout primitives --- */
.wrap { max-width: var(--content-width); margin: 0 auto; padding-left: var(--pad-x); padding-right: var(--pad-x); }
.wrap--narrow { max-width: 900px; }

.section { padding: var(--section-y) 0; border-top: 1px solid var(--line-12); }
.section--flush { border-top: 0; }
.section--alt { background: var(--bg-alt); }
.section--sunken { background: var(--bg-sunken); }
.section--light { background: var(--light); color: var(--on-light-body); }
.section--light h1, .section--light h2, .section--light h3 { color: var(--on-light); }

.section--light .eyebrow { color: var(--on-light-meta); }
.section--light .meta-mono { color: var(--on-light-meta); }

.pair {
  display: grid; grid-template-columns: 1fr 1fr; gap: 40px 56px; align-items: start;
}
.stack-gap > * + * { margin-top: 1rem; }

/* hairline-grid: children separated by 1px rules, no individual borders */
.rule-grid { display: grid; gap: 1px; background: var(--line-12); border: 1px solid var(--line-12); }
.rule-grid > * { background: var(--bg); }

/* --- Watermark ---
   The mark is anchored to the centred content column (.hero__inner /
   .wm-anchor), not the full-bleed section, so on wide screens it stays next to
   the content instead of drifting to the viewport edge. The section still
   clips the bleed via overflow:hidden. */
.watermark-host { position: relative; overflow: hidden; }
.watermark {
  position: absolute; color: var(--light);
  opacity: .05; pointer-events: none;
}
.watermark svg { width: 100%; height: 100%; }
.watermark-host > *:not(.watermark):not(.wm-anchor) { position: relative; z-index: 1; }

/* zero-size layer aligned with the content column; children position against it */
.wm-anchor {
  position: absolute; top: 0; bottom: 0; left: 50%;
  width: min(100%, var(--content-width));
  transform: translateX(-50%);
  pointer-events: none; z-index: 0;
}

/* decorative vertical grid for the hero */
.grid-lines {
  position: absolute; inset: 0; z-index: 0; pointer-events: none;
  background-image: linear-gradient(90deg, rgba(217,217,217,.06) 1px, transparent 1px);
  background-size: 110px 100%;
}

/* --- Buttons & links --- */
.btn {
  display: inline-flex; align-items: center; gap: 14px;
  font: 400 15px/1 var(--font-text);
  padding: 16px 24px; border-radius: var(--radius-sm);
  border: 1px solid transparent; cursor: pointer;
  transition: background var(--dur) var(--ease), border-color var(--dur) var(--ease), color var(--dur) var(--ease);
}
.btn--primary { background: var(--light); color: var(--on-light); }
.btn--primary:hover { background: var(--fg-display); color: var(--on-light); }
.btn--secondary { background: transparent; border-color: var(--line-28); color: var(--fg); }
.btn--secondary:hover { border-color: var(--accent); color: var(--accent); }
.btn--block { display: flex; justify-content: space-between; width: 100%; }

.link-fwd {
  display: inline-block; font: 400 15px/1 var(--font-text);
  color: var(--accent); border-bottom: 1px solid var(--accent-line);
  padding-bottom: 4px;
}
.link-fwd:hover { color: var(--accent-hover); }

/* --- Chips --- */
.chip {
  display: inline-block; font: 400 12px/1 var(--font-mono);
  color: var(--fg-body); border: 1px solid var(--line-22);
  padding: 7px 10px; border-radius: var(--radius-sm);
}
.chip--accent { color: var(--accent); border-color: var(--accent-line); background: var(--accent-tint); }
.chip--selected { background: var(--light); color: var(--on-light); border-color: var(--light); }

/* --- Utilities --- */
.flow > * + * { margin-top: 22px; }
.flex-between { display: flex; align-items: baseline; justify-content: space-between; gap: 40px; }
.flex-wrap { display: flex; flex-wrap: wrap; gap: 10px; }
[hidden] { display: none !important; }
.visually-hidden {
  position: absolute; width: 1px; height: 1px; margin: -1px; padding: 0;
  overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0;
}

@media (max-width: 900px) {
  :root { --pad-x: var(--pad-x-mobile); --section-y: var(--section-y-mobile); }
  .pair { grid-template-columns: 1fr; gap: 20px; }
  .flex-between { flex-direction: column; align-items: flex-start; gap: 16px; }
}

@media (prefers-reduced-motion: reduce) {
  * { transition-duration: 0ms !important; scroll-behavior: auto !important; }
}