Universal Design (UD) is defined by the ISO as the "design of products, environments, programs and services to be usable by all people, to the greatest extent possible, without the need for adaptation or specialized design". UD considers accessibility for users of all needs, abilities, and age. However, making systems and products more accessible to users does not mean that they are usable. Good UD ensures both usability and accessibility (Powlik & Karshmer, 2002). Stephanidis (2001) states that universal access should be designed into the system rather than added on after the fact.
There are several sets of guidelines that help to standardize principles of UD (see examples below).
Seven Principles of Universal Design developed by the Center for Universal Design at North Carolina State University:
Equitable Use: The design is useful and marketable to people with diverse abilities.
Flexibility in Use: The design accommodates a wide range of individual preferences and abilities.
Simple and Intuitive Use: Use of the design is easy to understand, regardless of the user's experience, knowledge, language skills, or current concentration level.
Perceptible Information: The design communicates necessary information effectively to the user, regardless of ambient conditions or the user's sensory abilities.
Tolerance for Error: The design minimizes hazards and the adverse consequences of accidental or unintended actions.
Low Physical Effort: The design can be used efficiently and comfortably and with a minimum of fatigue.
Size and Space for Approach and Use: Appropriate size and space is provided for approach, reach, manipulation, and use regardless of user's body size, posture, or mobility.
Web Content Accessibility Content (WCAG):
This collection of guidelines for web accessbility are standards typically used by web developers to make websites more accessible. These guidelines are organized into four princples: perceivable, operable, understandable, robust.
References:
“How to Meet WCAG (Quick Reference).” How to Meet WCAG (Quickref Reference), https://www.w3.org/WAI/WCAG21/quickref/.
ISO. Guide for addressing accessibility in standards. https://www.iso.org/obp/ui/#iso:std:iso-iec:guide:71:ed-2:v1:en
Powlik, J. J., & Karshmer, A. I. (2002). When accessibility meets usability. Universal Access in the Information Society, 1(3), 217–222. https://doi.org/10.1007/s10209-002-0020-z
Preiser, W. F. E., & Ostroff, E. (2001). Universal design handbook. New York: McGraw-Hill.
Stephanidis, C. (2001). Adaptive Techniques for Universal Access. User Modeling and User-Adapted Interaction, 11, 159-179.