/* main.css — pavingcalc.net 完整设计系统
   基于 Stripe 设计语言改造为工程行业风格：
   - Highway Blue 主交互色 (#1b56b4)
   - Asphalt Orange 警示/主 CTA (#ea6b22)
   - Deep Navy 标题 (#061b31)
   - 字体：自托管 Inter（在 critical.css 内联），mono 用系统栈
   异步加载，不影响首屏 */

/* ================================================
   1. 通用排版（继承 critical.css 的 token）
   ================================================ */

h2,
h3,
h4 {
  color: var(--c-heading);
  font-weight: 300;
  letter-spacing: -0.2px;
  margin: 2.5rem 0 1rem;
}

h2 {
  font-size: 1.5rem;
  line-height: 1.2;
  letter-spacing: -0.32px;
}

h3 {
  font-size: 1.2rem;
  line-height: 1.25;
  margin-top: 1.75rem;
  font-weight: 400;
}

h4 {
  font-size: 1rem;
  font-weight: 500;
  margin-top: 1.5rem;
}

@media (min-width: 1024px) {
  h2 {
    font-size: 2rem;
    letter-spacing: -0.5px;
  }
  h3 {
    font-size: 1.375rem;
  }
}

.lead {
  font-size: 1.1rem;
  color: var(--c-body);
  line-height: 1.6;
  margin-bottom: 1.5rem;
}

.key-sentence {
  display: block;
  font-weight: 500;
  margin: 1.2rem 0;
  padding: 12px 16px;
  border-left: 3px solid var(--c-link);
  background: var(--c-surface);
  color: var(--c-heading);
  border-radius: 0 var(--r-sm) var(--r-sm) 0;
}

code,
kbd,
samp {
  font-family: var(--ff-mono);
  font-size: 0.92em;
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: 3px;
  padding: 1px 6px;
}

pre {
  font-family: var(--ff-mono);
  background: #0d253d;
  color: #d6d9fc;
  padding: 16px 20px;
  border-radius: var(--r-md);
  overflow-x: auto;
  line-height: 1.7;
  font-size: 0.92rem;
}

pre code {
  background: transparent;
  border: 0;
  padding: 0;
  color: inherit;
}

ul,
ol {
  padding-left: 1.5rem;
  color: var(--c-label);
}

li {
  margin-bottom: 0.4rem;
}

hr {
  border: 0;
  border-top: 1px solid var(--c-border);
  margin: 32px 0;
}

blockquote {
  margin: 24px 0;
  padding: 16px 20px;
  background: var(--c-surface);
  border-left: 4px solid var(--c-cta);
  border-radius: 0 var(--r-md) var(--r-md) 0;
  color: var(--c-heading);
  font-style: normal;
}

blockquote cite {
  display: block;
  margin-top: 8px;
  font-size: 0.88rem;
  color: var(--c-body);
  font-style: normal;
}

/* ================================================
   2. Layout 容器
   ================================================ */

.container {
  max-width: var(--max-w);
  margin: 0 auto;
  padding: 0 20px;
}

.container--narrow {
  max-width: 760px;
}

.main {
  padding: 24px 0 64px;
}

.section {
  padding: 48px 0;
}

.section--dark {
  background: var(--c-dark);
  color: rgba(255, 255, 255, 0.85);
}

.section--dark h2,
.section--dark h3 {
  color: #fff;
}

.section--surface {
  background: var(--c-surface);
}

.grid {
  display: grid;
  gap: 20px;
}

@media (min-width: 640px) {
  .grid--2 {
    grid-template-columns: repeat(2, 1fr);
  }
}

@media (min-width: 1024px) {
  .grid--3 {
    grid-template-columns: repeat(3, 1fr);
  }
}

/* ================================================
   3. Breadcrumb
   ================================================ */

.breadcrumb {
  font-size: 0.85rem;
  padding: 12px 0 4px;
  color: var(--c-body);
}

.breadcrumb ol {
  list-style: none;
  padding: 0;
  margin: 0;
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
}

.breadcrumb li {
  margin: 0;
  display: inline-flex;
  align-items: center;
}

.breadcrumb li + li::before {
  content: '/';
  margin: 0 6px;
  color: var(--c-border);
}

.breadcrumb a {
  color: var(--c-body);
}

.breadcrumb [aria-current='page'] {
  color: var(--c-heading);
  font-weight: 500;
}

/* ================================================
   4. Reading progress + Author byline
   ================================================ */

.reading-progress {
  position: fixed;
  top: 0;
  left: 0;
  width: 0;
  height: 3px;
  background: linear-gradient(90deg, var(--c-link), var(--c-cta));
  z-index: 999;
  transition: width 0.1s linear;
  pointer-events: none;
}

.author-byline {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 16px 0;
  margin: 12px 0 24px;
  border-top: 1px solid var(--c-border);
  border-bottom: 1px solid var(--c-border);
}

.author-byline img {
  width: 48px;
  height: 48px;
  border-radius: 50%;
  flex-shrink: 0;
  border: 2px solid var(--c-border);
}

.author-byline > div {
  display: flex;
  flex-direction: column;
  font-size: 0.9rem;
  line-height: 1.4;
}

.author-byline a {
  font-weight: 500;
  color: var(--c-heading);
}

.author-byline span {
  color: var(--c-body);
  font-size: 0.85rem;
}

.author-byline time {
  color: var(--c-body);
  font-size: 0.8rem;
  margin-top: 2px;
}

/* ================================================
   5. Content preview / TOC / Section nav
   ================================================ */

.content-preview {
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
  padding: 20px 24px;
  margin: 24px 0;
}

.content-preview p {
  margin-bottom: 8px;
  font-weight: 500;
  color: var(--c-heading);
}

.content-preview ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.content-preview li {
  padding-left: 24px;
  position: relative;
  margin-bottom: 6px;
  color: var(--c-label);
}

.content-preview li::before {
  content: '✓';
  position: absolute;
  left: 0;
  top: 0;
  color: var(--c-success);
  font-weight: 700;
}

.article-nav {
  background: var(--c-bg);
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
  padding: 20px 24px;
  margin: 24px 0;
  box-shadow: var(--shadow-sm);
}

.article-nav h2,
.article-nav .article-nav__title {
  margin: 0 0 12px;
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 1px;
  color: var(--c-body);
  font-weight: 500;
}

.article-nav ol {
  list-style: decimal;
  padding-left: 1.4rem;
  margin: 0;
}

.article-nav li {
  margin-bottom: 6px;
}

.article-nav a {
  color: var(--c-link);
}

/* ================================================
   6. Buttons
   ================================================ */

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 12px 22px;
  border-radius: var(--r-sm);
  font-family: inherit;
  font-size: 16px;
  font-weight: 500;
  line-height: 1;
  border: 1px solid transparent;
  cursor: pointer;
  text-decoration: none;
  transition: background 0.15s, color 0.15s, border-color 0.15s, box-shadow 0.15s;
  min-height: 48px;
  min-width: 48px;
}

.btn:hover {
  text-decoration: none;
}

.btn--primary {
  background: var(--c-cta);
  color: #fff;
}

.btn--primary:hover {
  background: var(--c-cta-hover);
  color: #fff;
  box-shadow: var(--shadow-sm);
}

.btn--secondary {
  background: var(--c-link);
  color: #fff;
}

.btn--secondary:hover {
  background: var(--c-link-hover);
  color: #fff;
}

.btn--ghost {
  background: transparent;
  color: var(--c-link);
  border-color: #b9c8e0;
}

.btn--ghost:hover {
  background: rgba(27, 86, 180, 0.06);
  color: var(--c-link-hover);
}

.btn--block {
  display: flex;
  width: 100%;
}

.btn--sm {
  padding: 8px 14px;
  font-size: 14px;
  min-height: 40px;
}

/* ================================================
   7. Card / Surfaces
   ================================================ */

.card {
  background: var(--c-bg);
  border: 1px solid var(--c-border);
  border-radius: var(--r-lg);
  padding: 24px;
  box-shadow: var(--shadow-sm);
}

.card--feature {
  box-shadow: var(--shadow-md);
}

.card h3 {
  margin-top: 0;
}

/* ================================================
   8. Calculator widget — 核心组件
   ================================================ */

.calc {
  background: var(--c-bg);
  border: 1px solid var(--c-border);
  border-radius: var(--r-lg);
  box-shadow: var(--shadow-md);
  overflow: hidden;
  margin: 24px 0 40px;
}

.calc__tabs {
  display: flex;
  background: var(--c-surface);
  border-bottom: 1px solid var(--c-border);
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
}

.calc__tab {
  flex: 1 0 auto;
  padding: 14px 20px;
  background: transparent;
  border: 0;
  border-bottom: 3px solid transparent;
  font-family: inherit;
  font-size: 14px;
  font-weight: 500;
  color: var(--c-body);
  cursor: pointer;
  white-space: nowrap;
  min-height: 48px;
  transition: color 0.15s, border-color 0.15s, background 0.15s;
}

.calc__tab:hover {
  background: rgba(27, 86, 180, 0.04);
  color: var(--c-link);
}

.calc__tab[aria-selected='true'] {
  color: var(--c-heading);
  border-bottom-color: var(--c-cta);
  background: #fff;
}

.calc__body {
  padding: 24px;
}

.calc__unit-toggle {
  display: inline-flex;
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
  padding: 3px;
  margin-bottom: 20px;
  font-size: 13px;
  font-weight: 500;
}

.calc__unit-toggle button {
  background: transparent;
  border: 0;
  padding: 8px 16px;
  border-radius: var(--r-sm);
  cursor: pointer;
  color: var(--c-body);
  font-family: inherit;
  font-size: 13px;
  min-height: 36px;
}

.calc__unit-toggle button[aria-pressed='true'] {
  background: var(--c-link);
  color: #fff;
  box-shadow: rgba(27, 86, 180, 0.25) 0 4px 8px -2px;
}

.calc__shape {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
  margin-bottom: 20px;
}

.calc__shape button {
  background: var(--c-bg);
  border: 1px solid var(--c-border);
  padding: 10px 14px;
  border-radius: var(--r-sm);
  cursor: pointer;
  font-family: inherit;
  font-size: 13px;
  color: var(--c-label);
  min-height: 44px;
  display: inline-flex;
  align-items: center;
  gap: 6px;
}

.calc__shape button:hover {
  border-color: var(--c-link);
  color: var(--c-link);
}

.calc__shape button[aria-pressed='true'] {
  background: rgba(27, 86, 180, 0.08);
  border-color: var(--c-link);
  color: var(--c-link);
  font-weight: 500;
}

.calc__grid {
  display: grid;
  gap: 16px;
}

@media (min-width: 640px) {
  .calc__grid {
    grid-template-columns: repeat(2, 1fr);
  }
}

.calc__field {
  display: flex;
  flex-direction: column;
  gap: 6px;
}

.calc__field label {
  font-size: 13px;
  font-weight: 500;
  color: var(--c-label);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 6px;
}

.calc__field-hint {
  font-size: 12px;
  color: var(--c-body);
  font-weight: 400;
}

.calc__input-wrap {
  position: relative;
  display: flex;
  align-items: stretch;
}

.calc__input {
  flex: 1 1 auto;
  padding: 12px 64px 12px 14px;
  font-family: inherit;
  font-size: 16px;
  color: var(--c-heading);
  background: #fff;
  border: 1px solid var(--c-border);
  border-radius: var(--r-sm);
  min-height: 48px;
  width: 100%;
}

.calc__input:focus {
  outline: none;
  border-color: var(--c-link);
  box-shadow: 0 0 0 3px rgba(27, 86, 180, 0.18);
}

.calc__input-suffix {
  position: absolute;
  right: 12px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 13px;
  color: var(--c-body);
  font-family: var(--ff-mono);
  pointer-events: none;
}

.calc__presets {
  display: flex;
  gap: 6px;
  flex-wrap: wrap;
  margin-top: 6px;
}

.calc__presets button {
  background: transparent;
  border: 1px solid var(--c-border);
  padding: 6px 10px;
  border-radius: 12px;
  cursor: pointer;
  font-family: inherit;
  font-size: 12px;
  color: var(--c-body);
  min-height: 32px;
}

.calc__presets button:hover {
  border-color: var(--c-link);
  color: var(--c-link);
}

.calc__presets button[aria-pressed='true'] {
  background: rgba(27, 86, 180, 0.08);
  border-color: var(--c-link);
  color: var(--c-link);
  font-weight: 500;
}

.calc__select {
  padding: 12px 14px;
  font-family: inherit;
  font-size: 14px;
  color: var(--c-heading);
  background: #fff;
  border: 1px solid var(--c-border);
  border-radius: var(--r-sm);
  min-height: 48px;
}

.calc__actions {
  display: flex;
  gap: 12px;
  margin-top: 20px;
  flex-wrap: wrap;
}

/* 多 section 累加（自定义形状） */
.calc__sections {
  display: flex;
  flex-direction: column;
  gap: 12px;
  margin-bottom: 16px;
}

.calc__section-row {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr auto;
  gap: 8px;
  align-items: end;
}

.calc__section-row .calc__field label {
  font-size: 11px;
}

.calc__section-row input {
  padding: 10px 12px;
  font-size: 14px;
  min-height: 44px;
  border: 1px solid var(--c-border);
  border-radius: var(--r-sm);
}

.calc__section-remove {
  background: transparent;
  border: 1px solid var(--c-border);
  border-radius: var(--r-sm);
  cursor: pointer;
  padding: 8px 12px;
  color: var(--c-body);
  font-size: 18px;
  line-height: 1;
  min-height: 44px;
}

.calc__section-remove:hover {
  color: #c0392b;
  border-color: #c0392b;
}

/* ----------- 计算结果区 ----------- */

.calc__result {
  border-top: 1px solid var(--c-border);
  background: linear-gradient(180deg, var(--c-surface) 0%, #fff 100%);
  padding: 28px 24px;
}

.calc__result h3 {
  margin: 0 0 16px;
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 1px;
  color: var(--c-body);
  font-weight: 500;
}

.calc__output {
  display: grid;
  gap: 14px;
  grid-template-columns: repeat(2, 1fr);
}

@media (min-width: 640px) {
  .calc__output {
    grid-template-columns: repeat(4, 1fr);
  }
}

.calc__output-card {
  background: #fff;
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
  padding: 16px;
  box-shadow: var(--shadow-sm);
}

.calc__output-label {
  font-size: 11px;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  color: var(--c-body);
  font-weight: 500;
}

.calc__output-value {
  font-family: var(--ff-mono);
  font-feature-settings: 'tnum';
  font-size: 1.5rem;
  font-weight: 500;
  color: var(--c-heading);
  margin-top: 4px;
  line-height: 1.1;
  letter-spacing: -0.3px;
}

.calc__output-sub {
  font-size: 12px;
  color: var(--c-body);
  margin-top: 4px;
  font-family: var(--ff-mono);
  font-feature-settings: 'tnum';
}

.calc__output-card--cost {
  background: linear-gradient(135deg, rgba(234, 107, 34, 0.08), rgba(234, 107, 34, 0.02));
  border-color: rgba(234, 107, 34, 0.25);
  grid-column: span 2;
}

@media (min-width: 640px) {
  .calc__output-card--cost {
    grid-column: span 4;
  }
}

.calc__explainer {
  margin-top: 16px;
  padding: 14px 18px;
  background: rgba(27, 86, 180, 0.06);
  border-left: 3px solid var(--c-link);
  border-radius: 0 var(--r-md) var(--r-md) 0;
  font-size: 0.95rem;
  color: var(--c-heading);
}

.calc__breakdown {
  margin-top: 18px;
  width: 100%;
  border-collapse: collapse;
  font-size: 0.9rem;
}

.calc__breakdown th,
.calc__breakdown td {
  padding: 8px 12px;
  text-align: left;
  border-bottom: 1px solid var(--c-border);
}

.calc__breakdown th {
  color: var(--c-body);
  font-weight: 500;
  text-transform: uppercase;
  font-size: 11px;
  letter-spacing: 0.5px;
}

.calc__breakdown td {
  font-family: var(--ff-mono);
  font-feature-settings: 'tnum';
  color: var(--c-heading);
}

.disclaimer {
  margin-top: 20px;
  padding: 14px 18px;
  background: rgba(155, 104, 41, 0.08);
  border-left: 3px solid var(--c-warn);
  border-radius: 0 var(--r-sm) var(--r-sm) 0;
  font-size: 0.88rem;
  color: var(--c-label);
}

.disclaimer strong {
  color: var(--c-warn);
}

/* ================================================
   9. SVG 工程示意图
   ================================================ */

.diagram {
  margin: 24px auto;
  display: block;
  max-width: 480px;
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
  padding: 20px;
}

.diagram svg {
  width: 100%;
  height: auto;
}

.diagram text {
  font-family: var(--ff-mono);
  font-size: 12px;
  fill: var(--c-label);
}

/* ================================================
   10. Tables
   ================================================ */

.table-wrapper {
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
  margin: 20px 0;
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
}

table.data {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.92rem;
}

table.data th,
table.data td {
  padding: 10px 14px;
  text-align: left;
  border-bottom: 1px solid var(--c-border);
}

table.data th {
  background: var(--c-surface);
  color: var(--c-heading);
  font-weight: 500;
  font-size: 13px;
  text-transform: uppercase;
  letter-spacing: 0.4px;
}

table.data td:nth-child(n + 2),
table.data th:nth-child(n + 2) {
  text-align: right;
  font-family: var(--ff-mono);
  font-feature-settings: 'tnum';
}

table.data tr:last-child td {
  border-bottom: 0;
}

/* ================================================
   11. FAQ / Accordion
   ================================================ */

.faq {
  margin: 32px 0;
}

.faq__item {
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
  margin-bottom: 10px;
  background: #fff;
  overflow: hidden;
}

.faq__item summary {
  cursor: pointer;
  padding: 16px 20px;
  font-weight: 500;
  color: var(--c-heading);
  list-style: none;
  position: relative;
  padding-right: 48px;
  min-height: 56px;
  display: flex;
  align-items: center;
}

.faq__item summary::-webkit-details-marker {
  display: none;
}

.faq__item summary::after {
  content: '+';
  position: absolute;
  right: 18px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 22px;
  font-weight: 300;
  color: var(--c-link);
  transition: transform 0.15s;
}

.faq__item[open] summary::after {
  content: '−';
}

.faq__item summary:hover {
  background: var(--c-surface);
}

.faq__answer {
  padding: 0 20px 18px;
  color: var(--c-label);
}

.faq__answer p:last-child {
  margin-bottom: 0;
}

/* ================================================
   12. Related articles / Next reads
   ================================================ */

.related {
  margin: 48px 0 24px;
  padding: 28px 24px;
  background: var(--c-surface);
  border-radius: var(--r-lg);
}

.related h2 {
  margin-top: 0;
  font-size: 1.2rem;
  font-weight: 500;
}

.related ul {
  list-style: none;
  padding: 0;
  display: grid;
  gap: 10px;
}

@media (min-width: 640px) {
  .related ul {
    grid-template-columns: repeat(2, 1fr);
  }
}

.related li {
  margin: 0;
}

.related a {
  display: block;
  padding: 14px 18px;
  background: #fff;
  border: 1px solid var(--c-border);
  border-radius: var(--r-md);
  color: var(--c-heading);
  font-weight: 500;
  transition: border-color 0.15s, box-shadow 0.15s;
}

.related a:hover {
  border-color: var(--c-link);
  box-shadow: var(--shadow-sm);
  text-decoration: none;
  color: var(--c-link);
}

.related .related__meta {
  display: block;
  font-size: 12px;
  color: var(--c-body);
  font-weight: 400;
  margin-top: 4px;
}

/* ================================================
   13. Footer
   ================================================ */

.site-footer {
  background: var(--c-dark);
  color: rgba(255, 255, 255, 0.75);
  padding: 48px 0 24px;
  margin-top: 64px;
  font-size: 0.92rem;
}

.site-footer__grid {
  display: grid;
  gap: 32px;
  grid-template-columns: 1fr;
}

@media (min-width: 640px) {
  .site-footer__grid {
    grid-template-columns: 2fr 1fr 1fr 1fr;
  }
}

.site-footer h3 {
  color: #fff;
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 1.2px;
  margin: 0 0 12px;
  font-weight: 500;
}

.site-footer ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.site-footer li {
  margin-bottom: 8px;
}

.site-footer a {
  color: rgba(255, 255, 255, 0.75);
}

.site-footer a:hover {
  color: #fff;
}

.site-footer__brand p {
  color: rgba(255, 255, 255, 0.7);
  margin-top: 12px;
  line-height: 1.6;
  max-width: 38ch;
}

.site-footer__brand .brand {
  color: #fff;
}

.site-footer__brand .brand__mark {
  background: #fff;
}

.site-footer__brand .brand__mark::after {
  background: var(--c-cta);
}

.site-footer__bottom {
  margin-top: 36px;
  padding-top: 16px;
  border-top: 1px solid rgba(255, 255, 255, 0.1);
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 12px;
  font-size: 0.82rem;
  color: rgba(255, 255, 255, 0.55);
}

/* ================================================
   14. Badges / Status pills
   ================================================ */

.badge {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  padding: 2px 10px;
  border-radius: 12px;
  font-size: 11px;
  font-weight: 500;
  letter-spacing: 0.3px;
}

.badge--success {
  background: var(--c-success-bg);
  color: var(--c-success);
  border: 1px solid rgba(21, 190, 83, 0.4);
}

.badge--info {
  background: rgba(27, 86, 180, 0.12);
  color: var(--c-link);
  border: 1px solid rgba(27, 86, 180, 0.3);
}

.badge--warn {
  background: rgba(155, 104, 41, 0.14);
  color: var(--c-warn);
  border: 1px solid rgba(155, 104, 41, 0.35);
}

/* ================================================
   15. Methodology / Author bio specific
   ================================================ */

.bio-card {
  display: grid;
  gap: 24px;
  grid-template-columns: 1fr;
  background: linear-gradient(135deg, var(--c-surface), #fff);
  border: 1px solid var(--c-border);
  border-radius: var(--r-lg);
  padding: 28px;
  margin: 24px 0;
}

@media (min-width: 640px) {
  .bio-card {
    grid-template-columns: 160px 1fr;
    align-items: start;
  }
}

.bio-card img {
  width: 160px;
  height: 160px;
  border-radius: 50%;
  border: 4px solid #fff;
  box-shadow: var(--shadow-md);
}

.bio-card h2 {
  margin-top: 0;
}

.bio-card .expertise {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  margin-top: 12px;
}

/* ================================================
   16. Formula display block (Methodology page)
   ================================================ */

.formula {
  background: #0d253d;
  color: #b9c8e0;
  padding: 18px 22px;
  border-radius: var(--r-md);
  font-family: var(--ff-mono);
  font-size: 0.98rem;
  line-height: 1.9;
  margin: 18px 0;
  overflow-x: auto;
}

.formula em {
  color: #ffd7ef;
  font-style: normal;
}

.formula strong {
  color: #fff;
  font-weight: 500;
}

/* ================================================
   17. Helpers
   ================================================ */

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

.text-muted {
  color: var(--c-body);
}

.text-mono {
  font-family: var(--ff-mono);
  font-feature-settings: 'tnum';
}

.mt-0 { margin-top: 0; }
.mt-2 { margin-top: 16px; }
.mt-4 { margin-top: 32px; }
.mb-0 { margin-bottom: 0; }
.mb-2 { margin-bottom: 16px; }
.mb-4 { margin-bottom: 32px; }

.hidden { display: none !important; }

/* Mobile-first 优化：移动端字号、间距加大可读性 */
@media (max-width: 640px) {
  .calc__body {
    padding: 18px 16px;
  }
  .calc__result {
    padding: 20px 16px;
  }
  .calc__output-value {
    font-size: 1.3rem;
  }
  .section {
    padding: 32px 0;
  }
  h2 {
    margin-top: 2rem;
  }
}