/* Trang vở ô ly: lề đỏ, mực xanh, bút đỏ chấm bài. */
:root {
  --paper: #fbfcfe;
  --rule: #d3e0f0;
  --margin: #e5675c;
  --ink: #0f2c7a;      /* mực xanh bi, tương phản ~13:1 trên giấy */
  --ink-2: #2f4590;    /* chữ phụ, ~8:1 */
  --pen: #d1261b;      /* bút đỏ, chỉ dùng cho điểm nhấn */
  --u: 2rem;           /* một dòng kẻ */
  --margin-w: 0px;
  --gap: 1.25rem;
  --pad: 1rem;
  --font-display: "Bricolage Grotesque", "Be Vietnam Pro", "Noto Sans Thai", system-ui, sans-serif;
  --font-body: "Be Vietnam Pro", "Noto Sans Thai", system-ui, sans-serif;
  --font-hand: "Patrick Hand", "Be Vietnam Pro", cursive;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: 5rem; }
body {
  margin: 0; background: var(--paper); color: var(--ink);
  font: 400 1rem/var(--u) var(--font-body);
  -webkit-font-smoothing: antialiased;
}
@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } }

.sheet {
  overflow-x: clip; position: relative; max-width: 72rem; margin: 0 auto; min-height: 100vh;
  padding: 0 var(--pad) 0 calc(var(--pad) + 1.25rem);
  background-image: linear-gradient(var(--rule) 1px, transparent 1px);
  background-size: 100% var(--u);
  background-position: 0 calc(var(--u) - 1px);
}
/* lề đỏ */
.sheet::before {
  content: ""; position: absolute; top: 0; bottom: 0; left: calc(var(--pad) + 0.5rem + var(--margin-w));
  width: 2px; background: var(--margin); opacity: .85; pointer-events: none;
}
@media (min-width: 52rem) {
  :root { --margin-w: 12rem; --pad: 2rem; --gap: 2.5rem; }
  .sheet { padding-left: var(--pad); }
  .sheet::before { left: calc(var(--pad) + var(--margin-w) + var(--gap) / 2); }
}

a { color: inherit; }
:focus-visible { outline: 3px solid var(--pen); outline-offset: 3px; border-radius: 2px; }
.skip { position: absolute; left: -999px; top: 0; background: var(--ink); color: var(--paper); padding: .5rem 1rem; z-index: 10; }
.skip:focus { left: 0; }

/* bố cục hai cột: lề / nội dung */
.row, .hero, .foot { display: block; padding-bottom: calc(var(--u) * 2); }
.margin-col, .body-col { min-width: 0; }
.body-col { padding-left: 1.5rem; }
@media (min-width: 52rem) {
  .row, .hero, .foot { display: grid; grid-template-columns: var(--margin-w) minmax(0, 1fr); column-gap: var(--gap); }
  .body-col { padding-left: 0; }
}

/* thanh đầu trang */
.top {
  display: flex; align-items: flex-start; justify-content: space-between; flex-wrap: wrap;
  min-height: calc(var(--u) * 2); padding: .25rem 0 .25rem 1.5rem; position: relative;
}
@media (min-width: 52rem) { .top { padding-left: calc(var(--margin-w) + var(--gap)); } }
.brand { font: 800 1.125rem/1 var(--font-display); text-decoration: none; min-height: 44px; display: inline-flex; align-items: center; }
.nav {
  display: grid; grid-template-rows: 0fr; width: 100%; margin-top: .25rem;
  border-top: 2px solid transparent; transition: grid-template-rows .28s ease, border-color .28s ease;
}
.nav.open { grid-template-rows: 1fr; border-top-color: var(--ink); }
.nav-inner {
  display: flex; flex-direction: column; align-items: flex-start; min-height: 0; overflow: hidden;
  padding: .25rem 0 1rem; visibility: hidden; transition: visibility 0s .28s;
}
.nav.open .nav-inner { visibility: visible; transition-delay: 0s; }
@media (prefers-reduced-motion: reduce) { .nav, .nav-inner { transition: none; } }
.nav a, .lang { min-height: 44px; display: inline-flex; align-items: center; text-decoration: none; font-weight: 500; }
.nav a:hover { text-decoration: underline; text-underline-offset: 5px; text-decoration-thickness: 2px; }
.lang {
  font: 600 .9rem var(--font-body); color: var(--ink); background: none; border: 2px solid var(--ink);
  padding: 0 .9rem; border-radius: 4px; cursor: pointer; margin-top: .5rem; align-self: flex-start;
}
.lang:hover { background: var(--ink); color: var(--paper); }
.menu-btn { min-width: 44px; min-height: 44px; background: none; border: 0; color: var(--ink); cursor: pointer; display: inline-flex; align-items: center; justify-content: center; }
@media (min-width: 52rem) {
  .menu-btn { display: none; }
  .nav { display: block; width: auto; margin: 0; border-top: 0; }
  .nav-inner { flex-direction: row; align-items: center; gap: 1.5rem; padding: 0; overflow: visible; visibility: visible; }
  .top { align-items: center; }
  .lang { margin: 0 0 0 .5rem; }
}

/* tiêu đề */
h1, h2 { font-family: var(--font-display); margin: 0; color: var(--ink); }
h1 {
  font-weight: 800; font-size: clamp(2.5rem, 9vw, 3.5rem); line-height: calc(var(--u) * 2);
  letter-spacing: -0.02em; text-wrap: balance; padding-top: var(--u); margin-bottom: var(--u);
}
@media (min-width: 52rem) { h1 { font-size: clamp(3.5rem, 7vw, 5.5rem); line-height: calc(var(--u) * 3); } }
h2 { font-weight: 800; font-size: 1.5rem; line-height: var(--u); padding-top: var(--u); letter-spacing: -0.01em; }
@media (min-width: 52rem) { h2 { padding-top: var(--u); } }

.lead { font-size: 1.125rem; max-width: 42rem; margin: 0 0 var(--u); color: var(--ink); }
.pen { position: relative; display: inline-block; white-space: nowrap; font-weight: 600; padding: 0 .7em; margin: 0 .05em 0 -.7em; }
.pen svg {
  position: absolute; left: -.15em; top: -.35em; width: calc(100% + .3em); height: calc(100% + .7em);
  overflow: visible; fill: none; stroke: var(--pen); stroke-width: 2.5; stroke-linecap: round; pointer-events: none;
}
.pen--h1 { padding: 0 .1em; margin: 0 .02em 0 .02em; font-weight: inherit; }
.pen--h1 svg { left: -.12em; top: -.12em; width: calc(100% + .24em); height: calc(100% + .24em); stroke-width: 3; }
.pen path { stroke-dasharray: 1; stroke-dashoffset: 1; animation: draw .9s .5s cubic-bezier(.5,0,.2,1) forwards; vector-effect: non-scaling-stroke; }
@keyframes draw { to { stroke-dashoffset: 0; } }
@media (prefers-reduced-motion: reduce) { .pen path { animation: none; stroke-dashoffset: 0; } }

/* nút và liên kết: định nghĩa một lần, dùng lại */
.cta-row { display: flex; flex-wrap: wrap; align-items: center; gap: .5rem 1.5rem; margin: 0; }
.btn {
  display: inline-flex; align-items: center; min-height: 44px; padding: 0 1.25rem; border-radius: 4px;
  background: var(--ink); color: var(--paper); font-weight: 600; text-decoration: none;
  border: 2px solid var(--ink); transition: background-color .15s, color .15s, transform .15s;
}
.btn:hover { background: var(--paper); color: var(--ink); }
.btn:active { transform: translateY(1px); }
.textlink { font-weight: 500; text-underline-offset: 5px; text-decoration-thickness: 2px; display: inline-flex; min-height: 44px; align-items: center; }
.textlink:hover { color: var(--pen); }

/* nội dung các mục */
.intro { font-size: 1.125rem; font-weight: 500; max-width: 40rem; margin: 0 0 var(--u); padding-top: var(--u); }
.body-col p { max-width: 40rem; margin-top: 0; margin-bottom: var(--u); }
.steps { margin: 0 0 var(--u); max-width: 40rem; }
.steps > div { display: grid; grid-template-columns: 5.5rem 1fr; gap: 0 1rem; margin-bottom: var(--u); }
@media (min-width: 40rem) { .steps > div { grid-template-columns: 7rem 1fr; } }
.steps dt { font: 800 1.25rem/var(--u) var(--font-display); }
.steps dd { margin: 0; }
.stack-note { color: var(--ink-2); }
.links { list-style: none; padding: 0; margin: 0 0 var(--u); }
.links a { font-weight: 600; font-size: 1.125rem; text-underline-offset: 5px; text-decoration-thickness: 2px; display: inline-flex; min-height: 44px; align-items: center; }
.links a:hover { color: var(--pen); }
.foot p { color: var(--ink-2); font-size: .9rem; margin: 0; padding-bottom: var(--u); }


/* trang 404: chữ viết tay nhỏ ở lề */
.note-404 { font: 400 1.25rem/1.3 var(--font-hand); color: var(--pen); margin: var(--u) 0 0; transform: rotate(-3deg); }

.play-list { list-style: none; padding: 0; margin: 0 0 var(--u); max-width: 40rem; }
.play-list li { margin-bottom: var(--u); }
.play-list strong { font-family: var(--font-display); font-weight: 800; display: block; }
.play-list span { color: var(--ink-2); }

.ghost { color: var(--ink-2); font-family: var(--font-hand); font-size: 1.25rem; margin-top: var(--u); }
.play-list a { color: inherit; text-decoration: none; }
.play-list a:hover strong { text-decoration: underline; text-underline-offset: 5px; text-decoration-thickness: 2px; }

/* hiệu ứng cuộn: chỉ bật khi JS thêm .js-reveal (không có JS thì nội dung hiện sẵn) */
.js-reveal .rv { opacity: 0; }
.js-reveal .rv.in { opacity: 1; animation-duration: .75s; animation-delay: var(--d, 0s); animation-fill-mode: both; }
.js-reveal .rv-stamp { transform-origin: left center; }
.js-reveal .rv-stamp.in { animation-name: rv-stamp; animation-timing-function: ease-out; }
.js-reveal .rv-drop.in  { animation-name: rv-drop;  animation-timing-function: cubic-bezier(.34, 1.56, .64, 1); }
.js-reveal .rv-left.in  { animation-name: rv-left;  animation-timing-function: cubic-bezier(.34, 1.56, .64, 1); }
.js-reveal .rv-right.in { animation-name: rv-right; animation-timing-function: cubic-bezier(.34, 1.56, .64, 1); }
.js-reveal .rv-spin.in  { animation-name: rv-spin;  animation-timing-function: cubic-bezier(.34, 1.56, .64, 1); }
.js-reveal .rv-wiggle.in { animation-name: rv-wiggle; animation-timing-function: ease-out; }
@keyframes rv-stamp  { 0% { opacity: 0; transform: scale(2.8) rotate(-14deg); } 60% { opacity: 1; transform: scale(.9) rotate(2deg); } 100% { opacity: 1; transform: none; } }
@keyframes rv-drop   { from { opacity: 0; transform: translateY(-70px) rotate(-4deg); } to { opacity: 1; transform: none; } }
@keyframes rv-left   { from { opacity: 0; transform: translateX(-90px) rotate(-6deg); } to { opacity: 1; transform: none; } }
@keyframes rv-right  { from { opacity: 0; transform: translateX(90px) rotate(6deg); } to { opacity: 1; transform: none; } }
@keyframes rv-spin   { from { opacity: 0; transform: rotate(-360deg) scale(.2); } to { opacity: 1; transform: none; } }
@keyframes rv-wiggle { 0% { opacity: 0; transform: scale(.5); } 40% { opacity: 1; transform: rotate(-8deg) scale(1.1); } 60% { transform: rotate(6deg); } 80% { transform: rotate(-3deg); } 100% { opacity: 1; transform: none; } }

/* hiệu ứng mở trang: chạy một lần, tắt nếu người dùng chọn giảm chuyển động */
@media (prefers-reduced-motion: no-preference) {
  .hero h1 { transform-origin: left center; animation: hero-slam .85s cubic-bezier(.34, 1.56, .64, 1) both; }
  .pen--h1 { animation: hero-pop .7s .55s cubic-bezier(.34, 1.56, .64, 1) both; }
  .pen--h1 path { animation-delay: 1.2s; }
  .hero .lead { animation: hero-drop .75s .4s cubic-bezier(.34, 1.56, .64, 1) both; }
  .hero .cta-row { animation: rv-wiggle .9s 1s ease-out both; transform-origin: left center; }
}
@keyframes hero-slam { from { opacity: 0; transform: translateX(-90px) skewX(-14deg); } to { opacity: 1; transform: none; } }
@keyframes hero-pop  { from { opacity: 0; transform: scale(0) rotate(-40deg); } to { opacity: 1; transform: none; } }
@keyframes hero-drop { from { opacity: 0; transform: translateY(-60px) rotate(-3deg); } to { opacity: 1; transform: none; } }

/* hiệu ứng header: fill-mode backwards để sau khi chạy xong vẫn rê chuột nghiêng được */
@media (prefers-reduced-motion: no-preference) {
  .brand { animation: hero-drop .7s .1s cubic-bezier(.34, 1.56, .64, 1) backwards; }
  .menu-btn { animation: hero-pop .6s .3s cubic-bezier(.34, 1.56, .64, 1) backwards; }
  .nav-inner > :nth-child(1) { animation: rv-wiggle .7s .25s ease-out backwards; }
  .nav-inner > :nth-child(2) { animation: rv-wiggle .7s .33s ease-out backwards; }
  .nav-inner > :nth-child(3) { animation: rv-wiggle .7s .41s ease-out backwards; }
  .nav-inner > :nth-child(4) { animation: rv-wiggle .7s .49s ease-out backwards; }
  .nav-inner > :nth-child(5) { animation: hero-pop .7s .57s cubic-bezier(.34, 1.56, .64, 1) backwards; }
  .brand, .nav a { transition: transform .15s ease; }
  .nav a:hover { transform: rotate(-4deg) scale(1.08); }
  .brand:hover { transform: rotate(-3deg) scale(1.05); }
}