HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>toolbar only-child</title>
  <style>
    body { font-family: sans-serif; padding: 16px; }
    .tool-group { display: inline-block; margin: 8px; padding: 8px; border: 1px solid #ccc; }
    button { margin: 0 2px; padding: 6px 10px; }
  </style>
</head>
<body>
  <div class="tool-group"><button>Save</button></div>
  <div class="tool-group">
    <button>Cut</button>
    <button>Copy</button>
    <button>Paste</button>
  </div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $(".tool-group button:only-child").css({
      borderRadius: "999px",
      padding: "8px 16px"
    });
  </script>
</body>
</html>

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.tool-group button:only-child toolbar (Try It)