To improve accessibility, WAI-ARIA provides Web developers with the option to add the following semantic information to Web pages and rich Internet widgets which are then exposed to the browser:
Roles to describe the type of widget presented, such as "menu", "treeitem", "slider" and "progressbar."
Roles to describe the structure of the Web page, such as headings, regions, search areas and navigation areas.
Properties to describe the state widgets are in, such as "checked" for a check box, "haspopup" for a menu that renders a sub-menu or other popup and "expanded/collapsed" for a tree node.
Properties to define live regions of a page that are likely to get updates (such as stock quotes), as well as an interruption policy for those updates. Assistive technologies may present critical updates as soon as they are rendered. However, incidental updates are presented only after completing the current task. For example, a screen reader informs a user of an incidental update only after it finishes reading the current paragraph.
Properties for drag-and-drop that describe drag sources and drop targets
A method to provide keyboard navigation for rich internet widgets.
The combination of these features and the structural information conveyed by the DOM structure allow authors to produce an interoperable solution to assistive technologies. (Source: WAI-ARIA Overview)
User Agent support for WAI-ARIA varies, but overall support for WAI-ARIA is improving. Browsers which support WAI-ARIA map WAI-ARIA roles and properties to platform accessibility APIs.
Firefox 1.5 and Firefox 2.0 partially supports WAI-ARIA, however it requires the use of namespaces, and doesn't support the use of Liveregions.
Firefox 3+ contains better support for WAI-ARIA, including Liveregions.
IE8 partially supports WAI-ARIA.
JAWS 8 and Window-Eyes 5.5+ partially support WAI-ARIA.
Jaws 10+ supports WAI-ARIA.
FireVox, a self-voicing extension to Firefox, also supports WAI-ARIA via direct DOM access.
NVDA partially supports WAI-ARIA.
Using technologies in an Accessibility Supported way is required for conformance claims. Read more about Accessibility Support. The WCAG Working Group plans to review which WAI-ARIA techniques are sufficient when Accessible Rich Internet Application specifications reach W3C recommendation status. Refer to WAI-ARIA Overview for the latest information on the status of WAI-ARIA.