Involving people with disabilities from the beginning of a project helps you better understand accessibility issues and implement more effective accessibility solutions. It also broadens your perspective in a way that can lead you to discover new ways of thinking about your product that will make it work better for more people in more situations.

This applies when designing and developing:
– Websites and web applications
– Browsers, media players, and assistive technologies
– Authoring tools such as content management systems (CMS), blog software, and WYSIWYG editors
– Accessibility standards and policies
– Web technologies and technical specifications, such as HTML

in this respect, the W3C WAI Web Accessibility Initiative (WAI) published documents to help:

The documents provide guidance on including real users, covering:
– How involving users early helps
– Finding a range of users
– Working with users
– Analyzing accessibility issues
– Drawing conclusions and reporting
– Note for usability professionals
– Combining user involvement with standards
Source: WAI Interest Group