:root {
	--canvas: #ffffff;
	--ink: #000000;
	--ink-2: rgb(0 0 0 / 64%);
	--ink-3: rgb(0 0 0 / 44%);
	--border: rgb(0 0 0 / 10%);
	--border-strong: rgb(0 0 0 / 18%);
	--hover: rgb(0 0 0 / 5%);
	--pressed: rgb(0 0 0 / 9%);
	--fill-quiet: rgb(0 0 0 / 4%);
	--radius: 8px;
	--max: 1080px;
	--measure: 640px;
}

* {
	box-sizing: border-box;
}

html {
	-webkit-text-size-adjust: 100%;
}

body {
	margin: 0;
	background: var(--canvas);
	color: var(--ink);
	font-family:
		'Inter',
		-apple-system,
		BlinkMacSystemFont,
		'Segoe UI',
		'Noto Sans TC',
		'PingFang TC',
		'Microsoft JhengHei',
		sans-serif;
	font-size: 16px;
	line-height: 1.6;
	font-weight: 400;
}

a {
	color: inherit;
}

.container {
	max-width: var(--max);
	margin: 0 auto;
	padding: 0 40px;
}

/* ---------- header ---------- */

.site-header {
	border-bottom: 1px solid var(--border);
}

.site-header .container {
	display: flex;
	align-items: center;
	gap: 16px;
	height: 64px;
}

.brand {
	display: flex;
	align-items: center;
	gap: 10px;
	text-decoration: none;
	font-weight: 500;
	font-size: 17px;
}

.brand img {
	width: 28px;
	height: 28px;
	display: block;
}

.site-header nav {
	margin-left: auto;
	display: flex;
	align-items: center;
	gap: 8px;
}

.nav-link {
	text-decoration: none;
	color: var(--ink-2);
	font-size: 14px;
	font-weight: 500;
	padding: 8px 12px;
	border-radius: 6px;
}

.nav-link:hover {
	color: var(--ink);
	background: var(--hover);
}

/* ---------- hero ---------- */

.hero {
	padding: 112px 0 96px;
}

.hero-kicker {
	font-size: 14px;
	font-weight: 500;
	color: var(--ink-3);
	margin: 0 0 20px;
}

.hero h1 {
	font-size: clamp(34px, 5vw, 52px);
	font-weight: 500;
	line-height: 1.12;
	letter-spacing: -0.02em;
	margin: 0 0 24px;
	max-width: 760px;
}

.hero h1 .dim {
	color: var(--ink-2);
}

.hero p {
	font-size: 18px;
	color: var(--ink-2);
	max-width: var(--measure);
	margin: 0 0 40px;
}

.cta-row {
	display: flex;
	flex-wrap: wrap;
	gap: 12px;
}

.btn {
	display: inline-block;
	text-decoration: none;
	font-size: 15px;
	font-weight: 500;
	padding: 12px 24px;
	border-radius: 8px;
	transition: background 0.15s ease;
}

.btn-primary {
	background: var(--ink);
	color: var(--canvas);
}

.btn-primary:hover {
	background: rgb(0 0 0 / 78%);
}

.btn-primary:active {
	background: rgb(0 0 0 / 64%);
}

.btn-secondary {
	color: var(--ink);
	background: var(--fill-quiet);
}

.btn-secondary:hover {
	background: var(--hover);
}

.btn-secondary:active {
	background: var(--pressed);
}

:focus-visible {
	outline: 2px solid var(--ink);
	outline-offset: 2px;
	border-radius: 4px;
}

/* ---------- chapters ---------- */

.chapter {
	padding: 96px 0;
}

.chapter + .chapter {
	border-top: 1px solid var(--border);
}

.chapter-heading {
	font-size: 13px;
	font-weight: 500;
	color: var(--ink-3);
	margin: 0 0 12px;
}

.chapter h2 {
	font-size: clamp(26px, 3.4vw, 34px);
	font-weight: 500;
	line-height: 1.2;
	letter-spacing: -0.015em;
	margin: 0 0 16px;
	max-width: 720px;
}

.chapter .lede {
	font-size: 17px;
	color: var(--ink-2);
	max-width: var(--measure);
	margin: 0 0 48px;
}

/* problem list */

.problem-list {
	list-style: none;
	margin: 0;
	padding: 0;
	max-width: 820px;
}

.problem-list li {
	padding: 20px 0;
	border-top: 1px solid var(--border);
	display: grid;
	grid-template-columns: 200px 1fr;
	gap: 24px;
}

.problem-list li strong {
	font-weight: 500;
}

.problem-list li p {
	margin: 0;
	color: var(--ink-2);
}

/* flow diagram */

.flow {
	display: grid;
	grid-template-columns: 1fr 1fr;
	gap: 24px;
	max-width: 820px;
}

.flow-step {
	border: 1px solid var(--border);
	border-radius: var(--radius);
	padding: 24px;
}

.flow-step h3 {
	font-size: 15px;
	font-weight: 500;
	margin: 0 0 8px;
}

.flow-step p {
	margin: 0;
	font-size: 15px;
	color: var(--ink-2);
}

.flow-step .step-no {
	display: block;
	font-size: 13px;
	color: var(--ink-3);
	margin-bottom: 12px;
}

.flow-main {
	border-color: var(--border-strong);
}

/* tables */

.table-wrap {
	max-width: 820px;
	overflow-x: auto;
}

table {
	width: 100%;
	border-collapse: collapse;
	font-size: 15px;
}

th,
td {
	text-align: left;
	padding: 12px 16px;
	border-bottom: 1px solid var(--border);
	vertical-align: top;
}

th {
	font-weight: 500;
	color: var(--ink-2);
	border-bottom-color: var(--border-strong);
}

td:first-child {
	white-space: nowrap;
}

td code,
p code,
li code {
	font-family: ui-monospace, SFMono-Regular, 'SF Mono', Menlo, Consolas, monospace;
	font-size: 0.875em;
	background: var(--fill-quiet);
	padding: 2px 6px;
	border-radius: 4px;
}

.note {
	max-width: 820px;
	font-size: 14px;
	color: var(--ink-3);
	margin: 20px 0 0;
}

/* footer */

.site-footer {
	border-top: 1px solid var(--border);
	padding: 48px 0 64px;
	margin-top: 48px;
}

.site-footer .container {
	display: flex;
	flex-wrap: wrap;
	gap: 12px 32px;
	align-items: center;
}

.site-footer p {
	margin: 0;
	font-size: 14px;
	color: var(--ink-3);
}

.site-footer .footer-links {
	display: flex;
	gap: 16px;
	margin-left: auto;
}

.site-footer a {
	font-size: 14px;
	color: var(--ink-2);
	text-decoration: none;
}

.site-footer a:hover {
	color: var(--ink);
	text-decoration: underline;
}

/* ---------- responsive ---------- */

@media (max-width: 720px) {
	.container {
		padding: 0 24px;
	}

	.hero {
		padding: 72px 0 64px;
	}

	.chapter {
		padding: 64px 0;
	}

	.problem-list li {
		grid-template-columns: 1fr;
		gap: 4px;
		padding: 16px 0;
	}

	.flow {
		grid-template-columns: 1fr;
		gap: 16px;
	}

	.site-footer .footer-links {
		margin-left: 0;
	}
}