HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>eq single element</title>

  <style>

    body { font-family: sans-serif; padding: 16px; }

    ul { margin: 12px 0; }

  </style>

</head>

<body>

  <ul class="nav">

    <li>List 1, item 1</li>

    <li>List 1, item 2</li>

    <li>List 1, item 3</li>

  </ul>

  <ul class="nav">

    <li>List 2, item 1</li>

    <li>List 2, item 2</li>

    <li>List 2, item 3</li>

  </ul>

  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>

  <script>

    $("ul.nav li:eq(1)").css("backgroundColor", "#ff0");

  </script>

</body>

</html>

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ul.nav li:eq(1) single match (Try It)