New HTML Technique(s?): Using HTML structural elements to identify landmark regions of a page

[carmacleod]

Add a new HTML technique similar to the one for ARIA11.

The relevant HTML elements are: main, header, footer, aside, form and section (nav already has its own technique, but it needs to be included in any list of landmark elements).

Alternatively (or in addition), add several, more detailed, HTML techniques - one for each of the HTML elements listed above, similar to the existing technique for nav: H97: Grouping related links using the nav element

If several are added, they will all have to link to each other, and to H97. If only one technique is added for all of them, please make sure that it links to H97.

ARIA11 should link to, and be linked from, the new HTML techniques.

[fstrr]

Working on this as part of #291, so closing this issue.