The Americans with Disabilities Act was instituted in 1990 in an effort to end discrimination based on differing abilities. Drawing heavily from the landmark Civil Rights Act of 1964, which established protections against discrimination based on race, religion, sex or national origin, the ADA went a step further by requiring organizations to provide “reasonable accommodations” to employees with disabilities.
Luckily, the international nonprofit World Wide Web Consortium (W3C) brings together smart people who develop open standards to ensure the long-term growth of the Web. They produce recommendations for making websites fair and equally accessible for the highest number of people possible. Their Web Content Accessibility Guidelines (WCAG) 2.1 offer clear guidance for web developers to achieve ADA compliance.
The ADA statute identifies who is a person with a disability, who has obligations under the ADA, general non-discrimination requirements and other basic obligations. It delegates fleshing out those obligations to federal agencies. The agencies issue regulations and design standards. The regulations have the details on the rights of people with disabilities and responsibilities of employers, state and local governments, transportation providers, businesses and non-profit organizations. The design standards specify how many entrances need to be accessible, how many toilet rooms and the design for those elements.  To know what the ADA requires, you need to read the law, regulations and design standards.  
People with disabilities that affect their sight, hearing, or mobility may have difficulty accessing certain parts of websites and other online properties unless certain accommodations are made. Just as businesses may need to make adjustments to their physical location so that disabled customers have easy access to the premises, companies may need to adjust certain aspects of their websites so individuals with disabilities can take full advantage of all the features and services.
Courts have essentially taken three positions when approached with this issue. Some courts take the position that the ADA applies to all commercial sites because the law was meant to protect disabled individuals from having a more difficult time than able-bodied individuals from doing business. That is why the website Scribd was unable to get a case summarily dismissed and ended up settling.