html5: Significance of attribute named required in checkbox/radio

Question

On form submission, how could you possibly mark a checkbox/radiobutton as required?

Source of inspiration: Pekka\'s answer to a question

Answer 1

I just tried it on a radio button in Firefox 4. Adding required to one radio input, then submitting before selecting one, triggers a "Please select one of these options" tooltip.

E.g. this works: