Websites should be accessible by all people regardless of ability or disability. The DDA (Disability Discrimination Act) refers to the practice of making websites usable by all people.
This includes people that have visually impairments, mobility issues, hearing impairments, prone to seizures and cognitive issues.
There are 3 different levels of accessibility with websites ranging from level 1 (A) through to level 3 (AAA). Each increasing stage requires further levels of validation to be achieved. The way that a website is designed, cut into HTML and programmed can affect each of the standards.