Today's web browsers and web engines have poor support for accessibility, hence visually or audibly impaired people have a hard time using web applications.
Currently there are several accessibility standards such as IAccessible2 and AT-SPI, so first step is to either establish which of these to use or write a bridge so one can have one API with multiple back-ends.

Some work has been done in this front, and it would make sense to unify efforts and provide something upstream (e.g. to freedesktop.org)

http://mindforks.blogspot.com/2008/04/safariwebkit-accessibility-news.html

Platform-specific notes;

Linux: Need a good AT-SPI D-Bus implementation.
Windows: Native accessibility interfaces might be too primitive, perhaps a viable option is IAccessible2 which might give us benefit of JAWS support with WebKit
Mac: Cocoa not yet supported in QAccessible. Blogosphere indicates cocoa accessibility support is under par but this is unconfirmed

See also

http://www.linuxfoundation.org/en/Accessibility/ATK/AT-SPI/AT-SPI_on_D-Bus