The Web offers the possibility of unprecedented access to information and interaction for many people with disabilities. That is, the accessibility barriers to print, audio, and visual media can be much more easily overcome through Web technologies.

The Web is an increasingly important resource in many aspects of life: education, employment, government, commerce, health care, recreation, and more. It is essential that the Web be accessible in order to provide equal access and equal opportunity to people with disabilities. An accessible Web can also help people with disabilities more actively participate in society.

On 11 December 2008 W3C announced a new standard that will help Web designers and developers create sites that better meet the needs of users with disabilities and older users. Drawing on extensive experience and community feedback, Web Content Accessibility Guidelines (WCAG) 2.0 improves upon W3C's groundbreaking initial standard for accessible Web content, applies to more advanced technologies, and is more precisely testable.

Thus Web Designer mostly creating commercial prototype should follow Web Accessibility Guidelines.

For more details check below

Web Accessibility
ADA (Section 508)
Checklist for ADA Compliance
Tools for Accessibility