<fieldset>
<legend>Output format</legend>
<div>
<input type="radio" name="format" id="txt" value="txt" checked>
<label for="txt">Text file</label>
</div>
<div>
<input type="radio" name="format" id="csv" value="csv">
<label for="csv">CSV file</label>
</div>
[…]
</fieldset>