El artículo Using the fieldset and legend elements explica el uso correcto. La explicación se resumen en estos consejos:
When to use a fieldset and legend
You should use the <fieldset> and <legend> elements when:
- You have a single multiple choice question (using radio buttons or checkboxes).
- You have several questions relating to the same topic (like text boxes, or any other type of field).
You should not use the <fieldset> and <legend> when:
- You have a single form field that asks for a single piece of information.