Und meine Frage bezieht/ bezog sich darauf, inwiefern User-Agents dies tun?

http://www.maxability.co.in/tag/html-5-accessibility-2/:

»The <nav> tag of HTML 5 and the role=”navigation” of ARIA are used for the same purpose. The area of web page that contains A collection of navigational Elements (usually links) for navigating the document or related documents.

Screen Reader Support

HTML 5 NAV

• JAWS13 supports nav tag in all the 3 browsers.
• NVDA 2012.3 does not support HTML 5 nav tag in Internet explorer 9 and Google Chrome but announces as navigation landmark in Firefox 20.0.1
• Talkback with Google Chrome do not support the HTML 5 nav tag.

ARIA ROLE="NAVIGATION"

JAWS 13 supports navigation (role) in all the 3 browsers.
NVDA 2012.3 supports navigation (role) in all the 3 browsers.
Talkback on Android supports the navigation role.«

http://html5accessibility.com/:

»IE: provides no semantic information via accessibility APIs
Chrome: exposes element with a section role in IA2
Firefox: exposes as ARIA landmark role="navigation" via IA2 object attribute, and as a section role in IA2 (Unsure about the correctness of this mapping). Refer to HTML5 Accessibility Chops: section elements«

Daher die allgemeine Empfehlung, vorübergehend

<nav role="navigation">

zu schreiben. Für main gilt ja dasselbe:

<main role="main">

Mathias