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.
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Using the fieldset and legend elements is a post from CSS-Tricks
via Reme Le Hane