HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>only-of-type rejection</title>
  <style>
    body { font-family: sans-serif; padding: 16px; }
    .box { margin: 10px 0; padding: 8px; border: 1px solid #ccc; }
    #output { margin-top: 12px; padding: 12px; background: #e0f2fe; border: 1px solid #0284c7; }
  </style>
</head>
<body>
  <div class="box"><span>Only</span></div>
  <div class="box"><span>First</span><span>Second</span></div>
  <div id="output"></div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("span:only-of-type").css("fontWeight", "bold");
    $("#output").text("Matches → " + $("span:only-of-type").length);
  </script>
</body>
</html>

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span:only-of-type same-tag rejection (Try It)