Origem do Template: bootstrap3/layout/fieldset.html

<fieldset {% if fieldset.css_id %}id="{{ fieldset.css_id }}"{% endif %} 
    {% if fieldset.css_class or form_style %}class="{{ fieldset.css_class }} {{ form_style }}"{% endif %}
    {{ fieldset.flat_attrs|safe }}>
    {% if legend %}<legend>{{ legend|safe }}</legend>{% endif %}
    {{ fields|safe }} 
</fieldset>