Flash is really not the choice for accessability, and you will most likely find a lot of visually impared people will be more frustrated with Flash. If you choose flash and javascript, know that you will be restricting the visually impared user to Jaws/Window Eyes (Screen Reader), MS Windows, and Internet Explorer.
As an example, work is currently underway to make Macromedia's Flash technology accessible to people who use screen-reading software. While some positive progress has been made, it will be a considerable time before most users will benefit, and even then, Flash may be accessible only in certain specific circumstances. It is certainly wrong for web designers to assume that improvements in the accessibility of a technology mean that it can be used indiscriminately without regard for the principles of accessible web design.
Mouse-only interactions
Do not use interactive elements within your Flash movie that require the use of the mouse. The most common of these are drag-and-drop and double click. There is no keyboard alternative for either of these events. Flash elements, such as scroll bars and sliders, must include functionality that allows them to be used via a keyboard.
Do not require fine motor skills in order to navigate the movie. Do not move buttons that the user may want to select and make sure that navigation items are of a sufficient size. Because of the visual nature of the Flash authoring environment, it is almost entirely inaccessible to both screen reader and keyboard users. Although menu items, panels, and drawing tools are exposed to screen readers and can be accessed via the keyboard, it is impossible to use the tools to add elements to the Flash stage without using a mouse.
Finally, screen reader users will need to access Macromedia Flash content using the Microsoft Internet Explorer browser. This is the only browser with support for MSAA.
All accessible Flash content must be tested on the Microsoft Windows platform. While there have been recent improvements to the Apple Macintosh OS 10.4 release (Tiger), including a built in screen reader called VoiceOver, the Flash Player does not support this screen reader. All accessible Flash content must be tested using Microsoft Internet Explorer. At the time of publishing this document, Internet Explorer was still the only accessible browser available. The Mozilla Project has made some improvements with the Firefox Browser, and support for screen readers will soon be available. However, the version of the Flash Player that runs in Firefox is not yet accessible.
That is because the Adobe approach to accessibility is windows centric. I found that reading the Adobe product (flash & PDF) chapters in Web Accessibility: Web Standards and Regulatory Compliance. For example, Flash only talks to Microsoft Active Accessibility (MSAA) through IE. The Adobe view is that visually impaired people will only use one of five screen readers on a Windows box using IE.
There is a demo version of JAWS available which runs for 20 minutes before you need to restart your OS, great for Parallels or VPC users.
The Adobe definitive list of screen readers (ie ones that work with flash/PDFS) are: GW Micro Window Eyes 4.2 or later, Freedom Scientific JAWS 4.5, 6.1 or later, IBM Home Page Reader 3.04, Dolphin HAL 6.50, KDS PC Talker (Japan), Browsealoud.
Make Flash simpler for:
Hearing disabilities
Provide synchronized captions for any audio that conveys content
Photo epilepsy
Remove strobing content that flashes between 2 and 55 times per second
Motor disabilities
Ensure the Flash content is keyboard accessible
Do not require fine motor skills
Cognitive disabilities
Give users control over time sensitive content Provide easy to use controls and navigation schemes Be consistent Use the clearest, simplest language appropriate to the content
Low vision
Provide plenty of contrast. Allow the Flash content to scale to a larger size. Because Flash is based on vector objects (mathematically defined lines and shapes) rather than raster (pixels of differing colors) technology, most Flash content can be easily scaled to any size without distortion. Individuals with low vision may be able to interact with Flash content in ways not possible with HTML content
Blindness
Ensure screen reader accessibility or provide an accessible alternative Ensure keyboard accessibility Because of the audio capabilities of Flash, it can present content through audio, thus removing the need for a screen reader to extract audio content from the Flash movie. However, you must warn users of screen readers that a self voicing item will play, and give them the option of stopping the audio, but do not interfere with screen reader audio or keyboard commands Provide textual equivalents for all non-text elements that convey content or provide a function
For more information on creating Accessible Flash Content visit http://www.webaim.org/techniques/flash/
For information on VoiceOver visit http://www.456bereastreet.com/archive/200505/voiceover_and_safari_screen_reading_on_the_mac/
http://images.apple.com/accessibility/voiceover/pdf/VO_GettingStarted.pdf
Following is the comparison between various screen readers.
For Section 508 details visit http://www.section508.gov/
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