HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>only-child equivalence</title>
  <style>
    body { font-family: sans-serif; padding: 16px; }
    #output { margin-top: 12px; padding: 12px; background: #fef3c7; border: 1px solid #f59e0b; white-space: pre-line; }
  </style>
</head>
<body>
  <article><p>Solo paragraph</p></article>
  <article><p>First</p><p>Second</p></article>
  <section><p>Another solo</p></section>
  <div id="output"></div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    var only = $("p:only-child").length;
    var both = $("p:first-child:last-child").length;
    $("#output").text(":only-child → " + only + "\n:first-child:last-child → " + both);
  </script>
</body>
</html>

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p:only-child vs p:first-child:last-child (Try It)