This guide provides you a  description of accessibility with best practices for theme developers.

Web accessibility refers to the practice of making websites usable by people of all abilities and disabilities. When the sites are correctly designed, developed and edited, all users can have equal access for information and functionality.

The W3C's Web Accessibility Initiative (WAI) Introduction to Web Accessibility says Web accessibility means that people with disabilities can use the Web. More specifically, Web accessibility means that people with disabilities can perceive, understand, navigate, and interact with the Web, and  they can contribute to the Web. Web accessibility give benefits to others, including older people with changing abilities due to aging.