Add the element for a single-line text field.

<html>
<head><style>body {min-height: 300px;} input { font-size: 16px; } textarea { font-size: 16px; } </style></head>
<body>
<h3>Mailing Address</h3>
<input type="text">
</body>
</html>]]>

Make sure to pick <input type="text">.