The POUR principles are a great place to start. These principles define four qualities of an accessible experience and they are at the foundation of the Web Content Accessibility Guidelines (WCAG) an international standard for making web content accessible.
Perceivable content will make it possible for all your learners to see and hear the information.
Operable content will help all your learners navigate the information independently using their preferred tools.
Understandable content will support your learners’ understanding through a consistent and predictable design.
Robust content will work for your learners on a range of current and future technologies, including assistive technologies.