Most of us are blessed with excellent vision and gross, fine motor skills. Nonetheless, there is a section of our society, who are not so fortunate and therefore to access webpages or web applications they use a different set of accessibility options.
These principles are – Perceivable, Operable, Understandable & Robust. It is also known by the term POUR. Incidentally, the Web Accessibility Initiative of the World Wide Web Consortium has set the above mentioned WCAG guidelines after careful study. If a digital content fails in any of the said parameters then it is not considered 100% accessible for people with disabilities and other shortcomings.
Web developers therefore need to follow the rules of POUR strictly in order to make their content available to people with special needs, and the reason is they do not use traditional input and output devices such as mouse or LCD / LED monitors. For example, individuals with partial or complete blindness typically exploit screen readers while a few opt for refreshable Braille device.