HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>style input demo</title>
  <style>
    body { font-family: sans-serif; padding: 16px; max-width: 28rem; }
    fieldset { padding: 12px; border: 1px solid #cbd5e1; border-radius: 6px; }
  </style>
</head>
<body>
  <fieldset>
    <legend>Contact</legend>
    <label>Name</label>
    <input type="text">
    <label>Message</label>
    <textarea rows="2"></textarea>
    <label>Topic</label>
    <select><option>General</option><option>Support</option></select>
    <button type="button">Send</button>
  </fieldset>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("fieldset :input").css({
      width: "100%",
      boxSizing: "border-box",
      marginBottom: "8px",
      display: "block"
    });
  </script>
</body>
</html>

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Style fieldset :input uniformly (Try It)