Opera continues to be the best browser (IMO ;-) for accessibility testing with its new Opera 7 beta 1 release.
Opera7 ships with several default Style Sheets which directly benefit those testing accessibility issues or those who need accessibility features.
Default Styles Sheets include:
Emulate Text Browser Accessibility Layout Show images and links only High Contrast (white on black or black on white) Disable Tables
(and others)
These can be accessed via View > Styles or by clicking the DOWN ARROW in the Addressbar