org.danielnixon.saferdom.impl.html
The containing form element, if this element is in a form.
The containing form element, if this element is in a form. If the button is not a descendant of a form element, then the attribute can be the ID of any form element in the same document it is related to, or the null value if none matches.
MDN
https://html.spec.whatwg.org/multipage/forms.html#the-fieldset-element