Universal design is a collection of methods and theories combined together to create a principal framework when designing for people. Main goal of universal design theory is inclusion of people with various levels of abilities and disabilities, be it permanent, temporary or short-term, in the use of the final product. Offline, universal design principles are often used in architectural and interior design, and ergonomics. Some examples of designing for accessibility in real life would be translation of signs into Braille alphabet, installation of small fountains to help visually impaired people move around, ramps and lifts for wheelchairs. On the web, examples include using high contrast letters and colors, large fonts, captions. There are various methods available to evaluate the product at all development stages for its qualities of accessibility and usability, however taking into account these principals at the design stage is crucial and beneficial for all. If a product is hard to be used by a person with normal abilities, then that effect is 10x times higher for people with limited abilities.
References:
https://www.ted.com/talks/elise_roy_when_we_design_for_disability_we_all_benefit?language=en
https://www.ted.com/talks/teddi_doupe_diversity_inclusion_and_universal_design
Why Is Accessible Design Good for Everyone? | ARTiculations https://www.youtube.com/watch?v=wBSy7VrGQVU
The 7 Principles of Universal Design | Ed Roberts Campus https://www.youtube.com/watch?v=G-tHuD7R8cs