As the web becomes more and more interactive, developing for accessibility becomes more and more complex.
In the past, web developers had to worry relatively little about accessibility: when markup did little more than tell the browser how to format text, the browser could decide whether to format for the screen, or a reader, or whatever.
ARIA, the Accessible Rich Internet Applications Suite, is designed to help web developers make Web 2.0-type interactivity more accessible to everyone. The W3C describes ARIA this way:
ARIA reached full Candidate Recommendation status last January, so it's already being implemented.
Besides differing browser implementations, a multiplicity of screen readers adds another ingredient to the already-a-bit-complicated mix. The same screen reader may respond differently in different browsers, for example.
To increase your awareness of HTML5 accessibility support in modern browsers, using particular screen readers, Jason Kiss recently posted this helpful series of videos, showing various screen readers working with HTML5 sections and ARIA landmarks, in various browsers.
Should be quite useful for HTML5 developers coding for accessibility. Videos are only about 30 seconds each.