The nsIDOMHTMLHeadingElement interface is the interface to a
[X]HTML h1, h2, h3, … element.
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/