HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :nth-child(3n+1) Example</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
    }

    ul {
      list-style: none;
      padding: 0;
      max-width: 16rem;
      border: 1px solid #e2e8f0;
      border-radius: 0.5rem;
      overflow: hidden;
    }

    li {
      padding: 0.65rem 0.9rem;
      border-bottom: 1px solid #e2e8f0;
      font-size: 0.9375rem;
      color: #334155;
    }

    li:last-child {
      border-bottom: none;
    }

    li:nth-child(3n+1) {
      background: #dcfce7;
      border-left: 4px solid #16a34a;
      padding-left: 0.65rem;
    }
  </style>
</head>
<body>
  <h1>Pattern</h1>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
    <li>Item 5</li>
    <li>Item 6</li>
    <li>Item 7</li>
  </ul>
  <p><small>Matches items 1, 4, and 7.</small></p>
</body>
</html>

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CSS :nth-child() - 3n+1 Formula