Accessibility means access generally. Accessibility in web design means creating web pages that everyone can use, regardless of disability or special needs, use and benefit from everything within their environment.
According to Wikipedia:
Accessibility refers to the design of products, devices, services, or environments for people with disabilities. The concept of accessible design ensures both “direct access” (i.e. unassisted) and “indirect access” meaning compatibility with a person’s assistive technology (for example, computer screen readers).
In WordPress, there is a mission to make WordPress accessible to everyone including the more than one billion humans with disabilities. And regular progress shows that this mission will be achieved in near future.
After hearing so much about Accessibility in WordPress, I wanted to try it in my theme also. So I decided to make my free theme Simple Life accessible. I was new and I did not know anything about making theme accessible. After spending few hours learning about key elements under Accessibility, I started modifying my theme. Honestly speaking, it took some iterations to finally make my theme fully accessible but I learned a lot in the process and I am happy about that. I want to share some points which are important while developing accessible WordPress theme or plugin.