Users must be able to understand the content and operation of the interface.  The web pages should operate in predictable ways, and the content should be readable and easy to understand.

New York University

The Web Content Accessibility Guidelines (WCAG) is a global standard for web content accessibility, with documents explaining how to make web content more accessible for those with disabilities.  Web content refers to text, images, and sounds, as well as code or markup.  In the WCAG 2.0 and 2.1 Documents, there are guidelines included based on these four principles: “perceivable, operable, understandable, and robust.”

Web Content Accessibility Guidelines (WCAG)

https://www.w3.org/WAI/standards-guidelines/wcag/

Web Content Accessibility Guidelines

Information and user interface components must be presented to users in a way that they can perceive - interfaces should be designed so that it is easier for people to see and hear it, as well as providing alternatives for text, time-based media, and other content for people with disabilities.

WCAG - Perceivable

Users must be able to navigate through and operate the interface.  It cannot require an interaction that the user is unable to perform.  This means that designs should ways to help users navigate, and should include making all of the functionality available from a keyboard.

WCAG - Operable

WCAG - Understandable

Users must be able to interpret content reliably, including those using assistive technologies.

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