HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS nested counters</title>
  <style>
    body {
      font-family: sans-serif;
      padding: 1.5rem;
      line-height: 1.6;
    }

    .outline {
      counter-reset: chapter;
    }

    .outline h2 {
      counter-increment: chapter;
      counter-reset: section;
    }

    .outline h2::before {
      content: "Chapter " counter(chapter) ": ";
      color: #2563eb;
      font-weight: 600;
    }

    .outline h3 {
      counter-increment: section;
      margin-left: 1rem;
    }

    .outline h3::before {
      content: counter(chapter) "." counter(section) " ";
      color: #64748b;
      font-weight: 600;
    }
  </style>
</head>
<body>
  <article class="outline">
    <h1>Document Outline</h1>
    <h2>Basics</h2>
    <h3>Syntax</h3>
    <h3>Values</h3>
    <h2>Examples</h2>
    <h3>Headings</h3>
  </article>
</body>
</html>

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CSS counter-increment Property - Nested Counters