form [HTML5]

The form element that the input element is associated with (its form owner). The value of the attribute must be an id of a <form> element in the same document. If this attribute is not specified, this <input> element must be a descendant of a <form> element. This attribute enables you to place <input> elements anywhere within a document, not just as descendants of their form elements. An input can only be associated with one form.