The nsIDOMHTMLElement interface is the primary [X]HTML element
interface. It represents a single [X]HTML element in the document
tree.
This interface is trying to follow the DOM Level 2 HTML specification:
http://www.w3.org/TR/DOM-Level-2-HTML/
with changes from the work-in-progress WHATWG HTML specification:
http://www.whatwg.org/specs/web-apps/current-work/
Indicates that the element is not yet, or is no longer, relevant.
See http://www.whatwg.org/html5/#the-hidden-attribute.