The practice of making tools and technologies work well for users with physical challenges is called accessibility. This concern applies to websites, computers, software programs, game consoles, and even motor vehicles, furniture, and food packaging. Ultimately, any product or service that we want to make available to physically challenged users can be said to have accessibility objectives.
In the case of websites, accessibility is largely about creating content and functionality that users can access comfortably (sometimes with assistive technologies), and pages that can change in appearance to accommodate vision impairments, or even fine motor skill limitations.
Proper web accessibility is achieved with a variety of design and development techniques that apply to everything from color and font choices to the flow and structure of pages, as well as the use of special widgets that modify colors and text sizes as required by users.
Like virtually everything else in the web world, accessibility is not defined by a single standard and there are many ways to get it right. And quite a few ways to get it wrong. The two most relevant standards that govern accessibility requirements are the W3C WAI WCAG (Web Content Accessibility Guidelines by the World Wide Web Consortium's Web Accessibility Initiative) and Section 508 (by the US Government).