The HTML role attribute (or landmark role) is designed to let non-visual browsers know where they are.
Adding simple html5 semantic elements and their associated landmark role attributes will help improve the html and general accessibility of moodle e.g. <banner> , <header>, <section>, <nav role='navigation'>, <div role='main'> etc...
Adding the richer roles (e.g. aria-readonly, aria-required and aria-valuemax, aria-valuemin and aria-valuenow) gives non-visual browsers the ability to use forms and other rich interactive elements they might otherwise find problematic.
edit : Have attached a starter markup plan for a basic m2 page at http://demo.moodle.net/course/view.php?id=5