To make it easier for authors and developers to create accessible offerings, standards have been developed in the past that can be used as a guide.
WCAG (Web Content Accessibility Guidelines) is a standard for web design, most of whose content can be applied to e-learning design. The complete principles, guidelines, success criteria and techniques of the WCAG can be found on the page of the W3C itself: Web Content Accessibility Guidelines (WCAG) 2.0.