Web accessibility is often overlooked in web design and development. Even when I teach it it seems I just gloss over it, explain what it is but do not demand it in final projects. It is an error that I will make sure to correct in the future.
There are different types of roles, the most important and common of which are the html5 structure roles . Eg.
In addition, there are the landmark roles which are regions of the page intended as navigational landmarks:
The following is a list of roles, their definitions and uses.
In addition to all of the above, there is also a website dedicated to web accessibility matters which is the Web Accessibility Initiative which has valuable information about web accessibility and Arias.