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”.
Disabilities covered under the ADA can be physical (e.g., muscular dystrophy, dwarfism, etc.), sensory (e.g., blindness, deafness, deaf-blindness), or cognitive (e.g., Down Syndrome). In 2008, the Americans with Disabilities Amendment Act broadened the scope of how disability is legally defined: psychological, emotional, and physiological conditions are now included.
Many government services and activities are also provided on websites because the public is able to participate in them at any time of day and without the assistance of government personnel. Many government websites offer a low cost, quick, and convenient way of filing tax returns, paying bills, renewing licenses, signing up for programs, applying for permits or funding, submitting job applications, and performing a wide variety of other activities.
All of that was well and good in 1990, when the then-nascent Internet was not the ubiquitous presence in the lives of Americans that it is today. For example, retail shopping in-person at a mall in 1990 was booming, unlike today where online shopping has completely changed the game for retailers. As time and technologies evolved however, the Department of Justice (DOJ), the entity charged with enforcing the ADA, hinted but never definitively stated that Title III may indeed be applicable to websites.