* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
  line-height: 1.6;
  color: #333;
  background: #fff;
  padding: 20px;
  max-width: 800px;
  margin: 0 auto;
}

h1 {
  font-size: 2rem;
  margin-bottom: 1.5rem;
  line-height: 1.3;
}

h2 {
  font-size: 1.5rem;
  margin-top: 3rem;
  margin-bottom: 1rem;
  padding-bottom: 0.5rem;
  border-bottom: 2px solid #eee;
}

h3 {
  font-size: 1.2rem;
  margin-top: 2rem;
  margin-bottom: 0.75rem;
}

h4 {
  font-size: 1.1rem;
  margin-top: 1.5rem;
  margin-bottom: 0.5rem;
  font-weight: 600;
}

p {
  margin-bottom: 1rem;
}

strong {
  font-weight: 600;
}

em {
  font-style: italic;
}

ul, ol {
  margin: 1rem 0 1rem 1.5rem;
}

li {
  margin-bottom: 0.5rem;
}

/* Базовые блоки */
.intro {
  background: #f8f9fa;
  padding: 1.5rem;
  border-left: 4px solid #333;
  margin: 2rem 0;
}

.intro p {
  margin-bottom: 0.5rem;
}

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

.key {
  font-weight: 500;
  color: #333;
  margin: 1.5rem 0;
  padding: 1rem 1.5rem;
  background: #fff9e6;
  border-radius: 4px;
}

.key p {
  margin-bottom: 0.5rem;
}

.key ul {
  margin: 0.5rem 0 0 1.5rem;
}

.key li {
  margin-bottom: 0.4rem;
}

.example {
  border-left: 3px solid #666;
  padding-left: 1rem;
  margin: 1.5rem 0;
  color: #555;
}

/* Блок «Для тих, хто хоче глибше» */
.deep {
  background: #f8f9fa;
  padding: 1.5rem;
  border-left: 3px solid #999;
  margin: 1.5rem 0;
  border-radius: 0 4px 4px 0;
}

.deep-title {
  font-size: 0.85rem;
  font-weight: 600;
  color: #666;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  margin-bottom: 1rem;
}

.deep p {
  font-size: 0.95rem;
  color: #555;
  margin-bottom: 0.75rem;
}

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

/* Главное оглавление */
.main-toc {
  background: #f8f9fa;
  padding: 2rem;
  margin: 2rem 0;
  border-radius: 4px;
}

.main-toc h2 {
  margin-top: 0;
  border: none;
  font-size: 1.5rem;
  margin-bottom: 1.5rem;
}

.main-toc-list {
  list-style: none;
  padding-left: 0;
  margin: 0;
}

.main-toc-list > li {
  margin: 1.5rem 0;
  padding: 1rem;
  background: white;
  border-radius: 4px;
  border-left: 3px solid #666;
}

.main-toc-list > li > a {
  text-decoration: none;
  color: #333;
  font-size: 1.2rem;
  font-weight: 600;
  display: block;
  margin-bottom: 0.5rem;
}

.main-toc-list > li > a:hover {
  color: #000;
}

.main-toc-desc {
  color: #666;
  font-size: 0.95rem;
  line-height: 1.5;
  margin-bottom: 0.25rem;
}

.main-toc-meta {
  color: #999;
  font-size: 0.85rem;
  margin-top: 0.5rem;
}

/* Навигационные кнопки */
.nav-buttons {
  display: flex;
  gap: 1rem;
  margin: 3rem 0 2rem;
  padding: 1rem 0;
  border-top: 1px solid #eee;
}

.nav-button {
  padding: 0.5rem 1rem;
  background: #f8f9fa;
  border: 1px solid #ddd;
  border-radius: 4px;
  text-decoration: none;
  color: #333;
  font-size: 0.9rem;
}

.nav-button:hover {
  background: #e9ecef;
}

/* Ссылки */
a {
  color: #333;
  text-decoration: underline;
}

a:hover {
  color: #000;
}

/* Адаптивность */
@media (max-width: 768px) {
  body {
    padding: 15px;
  }

  h1 {
    font-size: 1.5rem;
  }

  h2 {
    font-size: 1.3rem;
  }

  h3 {
    font-size: 1.1rem;
  }

  .nav-buttons {
    flex-direction: column;
  }
  
  .intro {
    padding: 1rem;
  }
  
  .key {
    padding: 1rem;
  }
  
  .deep {
    padding: 1rem;
  }
  
  .main-toc {
    padding: 1.5rem;
  }
}