Making a product or service to be available to as many people as possible, including people with disabilities or special needs defines the Accessibility of the product. Another term that is used with Accessibility is Assistive technologies. Assistive technologies are devices that support people with disabilities.
Web accessibility ensure that websites are built using Semantic HTML, proper tags for images, use of link names, forms with proper markup that facilitate text to speech software, typing using Braille for blind. Use of colours that can be recognised by colour blind people, use of images and simple english to support people with learning disabilities like dyslexia, appropriate captions & transcripts to support deaf people watching videos or other audio on the site are some of the other accessibility features.
Accessibility is a very important consideration for Learning & Education portals, Government funded initiatives, NGOs.
Building Accessibility could be complicated but with Drupal 7, accessibility was part of the platform. Understanding the accessibility features in Drupal will help you work along with it and benefit from it. Drupal core is committed to conform with the World Wide Web Consortium (W3C) guidelines: WCAG 2.0 and ATAG 2. Checkout Drupal Accessibility pledge: http://drupal.org/about/accessibility.
Drupal 8 development cycle core changes need to pass through Core Gates to ensure Quality & one among 5 gates being Accessibility Gate. Everett was the Drupal Core accessibility maintainer, Currently Mgifford is the Accessibility maintainer.
Checkout the Accessibility handbook and a Theming Guide Accessibility section for more details on Drupal & Accessibility. Become a member at http://groups.drupal.org to be uptodate with Drupal Accessibililty.
Do you want to do a quick review of how accessible your website is, check it out at: http://wave.webaim.org
May 9th is the global Accessibility awareness day! Learn more about it at: http://www.globalaccessibilityawarenessday.org/
Read Joe Devon's Blog that inspired Accessibility.