Accessibility in Web development means enabling as many people as possible to use websites, even when those people's abilities are limited in some way.
"Accessibility is most often used to describe facilities or amenities to assist people with disabilities, as in 'wheelchair accessible'. This can extend to Braille signage, wheelchair ramps, audio signals at pedestrian crossings, walkway contours, website design, and so on." 
"The Web is fundamentally designed to work for all people, whatever their hardware, software, language, culture, location, or physical or mental ability. When the Web meets this goal, it is accessible to people with a diverse range of hearing, movement, sight, and cognitive ability." 
Documentation about accessibility
- Web development
- A collection of articles designated to highlight web development issues in the world of accessibility
- A collection of articles to learn how to use ARIA to make your HTML documents more accessible.
- Assistive technology (AT) development
- A collection of articles intended for AT developers
Getting Community help
- View Mozilla forums...
Accessibility Tools for web developers