A polyfill for a proposed HTML Standard "outline algorithm".

The basic gist of the proposal is as follows:

• Focus on headings alone and leave landmarks to their own UI.
• h2-h6 always have heading level 2-6, unless their parent is hgroup, in which case hgroup is the heading and h2-h6 are just content.
• h1 and hgroup have heading level 1 + the number of ancestor sectioning content / sectioning root elements. (We include sectioning roots as it seems sensible that a h1 inside a blockquote should have a higher level than the one in the section or article the blockquote is in. A wrinkle here is that sectioning roots are not considered by the rendering section at the moment.)
• hgroup presents itself as a heading (role=heading) and its h1-h6 children as ordinary content (no role, like span).

Motivation can be found in HTML Standard issue 83.