HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>only-child lists</title>
  <style>
    body { font-family: sans-serif; padding: 16px; }
    li { margin: 4px 0; padding: 4px 8px; }
    #output { margin-top: 12px; padding: 12px; background: #e0f2fe; border: 1px solid #0284c7; }
  </style>
</head>
<body>
  <ul id="solo"><li>Only item</li></ul>
  <ul id="multi"><li>A</li><li>B</li><li>C</li></ul>
  <div id="output"></div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("li:only-child").css("backgroundColor", "#dbeafe");
    $("#output").text("Matches → " + $("li:only-child").length);
  </script>
</body>
</html>

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li:only-child single-item lists (Try It)