ARIA is a set of special accessibility attributes which can be added to any markup, but is especially suited to HTML. The role attribute defines what the general type of object is (such as an article, alert, or slider). Additional ARIA attributes provide other useful properties, such as a description for a form or the current value of a progressbar.
ARIA is implemented in most popular browsers and screen readers. However, implementations vary and older technologies don't support it well (if at all). Use either "safe" ARIA that degrades gracefully, or ask users to upgrade to newer technology.
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Yahoo! has done an amazing job of advancing ARIA here, by exercising ARIA's full capabilities and sharing their techniques. Yahoo! Search uses a combination of ARIA landmarks, live regions, and widgets.
Like the W3C WAI-ARIA specification, the official best practices represents a future ideal -- a day when authors can rely on consistent ARIA support across browsers and screen readers. The W3C documents provide an in-depth view of ARIA.
For now, web developers implementing ARIA should maximize compatibility. Use best practices docs and examples based on current implementations.
The community needs a complete set of WCAG techniques for WAI-ARIA + HTML, so that organizations can be comfortable claiming their ARIA-enabled content is WCAG compliant. This is mostly important when regulations or policies are based on WCAG.