Allow for the possibility of user agents implementing landmark navigation

[carmacleod]

The linked draft PR adds a second example to the description of Sequential Focus Navigation Starting Point, so that the Example box reads:

Example: For example, the user agent could set it to the position of the user's click if the user clicks on the document contents. Or, the user agent could set it to a target landmark element if the user agent has a mechanism for navigating by landmarks.

Of course, we would need to define "landmark element" and "navigating by landmarks" before this new sentence could go in.
Those definitions would go something like this:

• landmark element: An HTML element that has an implicit ARIA landmark role. These elements are: header (in body scope), footer (in body scope), main, nav and aside. (And hopefully also section and form with an associated label element, and search).
• navigating by landmarks: A user agent may provide a mechanism (for example, in a dialog box or through keyboard commands) for navigating to landmark elements. Keyboard commands could navigate to the landmark element that follows or precedes the Document's sequential focus navigation starting point, which would then be moved accordingly. One of the goals of providing landmark navigation would be to enable efficient navigation based on the author's structural organization of the Document.

Please help me figure out where in the spec these definitions would belong.
It would be really cool to create a new content category for "Landmark content" that intersects with Sectioning content and Flow content.

[domenic]

My main concern here is duplicating content between HTML and ARIA. If ARIA already defines implicit landmark roles, then we should link to its definitions, instead of creating our own.

Alternately, if we wanted to define landmark roles and the navigation thereof in HTML, that would be reasonable, but I don't think spreading it out over two specs would work that well. And the relevant expertise probably lies with the ARIA spec editors, for longer-term maintenance.

[botic]

I think my 12 year old Firefox bug report goes in the same direction: https://bugzilla.mozilla.org/show_bug.cgi?id=670928