Many people with disabilities use assistive technology that enables them to use computers. Some assistive technology involves separate computer programs or devices, such as screen readers, text enlargement software, and computer programs that enable people to control the computer with their voice. Other assistive technology is built into computer operating systems. For example, basic accessibility features in computer operating systems enable some people with low vision to see computer displays by simply adjusting color schemes, contrast settings, and font sizes. Operating systems enable people with limited manual dexterity to move the mouse pointer using key strokes instead of a standard mouse. Many other types of assistive technology are available, and more are still being developed.

Absent the adoption of specific technical requirements for websites through rulemaking, public accommodations have flexibility in how to comply with the ADA’s general requirements of nondiscrimination and effective communication. Accordingly, noncompliance with a voluntary technical standard for website accessibility does not necessarily indicate noncompliance with the ADA.
Basically, there are standards which apply to US federal agencies, and State or Local Government websites (not business/personal websites). While some disability discrimination laws do mention websites, they don’t specify the technical standards required to comply with the law. Regardless of ADA’s lack of stating websites must be fully accessible, the Department of Justice seems to rule on the side of “they should be”.
State and local governments will often post documents on their websites using Portable Document Format (PDF). But PDF documents, or those in other image based formats, are often not accessible to blind people who use screen readers and people with low vision who use text enlargement programs or different color and font settings to read computer displays.
Develop a plan for making your existing web content accessible. Describe your plan on an accessible webpage, and encourage input on how accessibility can be improved. Let visitors to your website know about the standards or guidelines that you are using to make your website accessible. When setting timeframes for accessibility modifications to your website, make more popular webpages a priority.