For federal institutions, Section 508 makes it very clear that all federal-related websites must be accessible to all individuals, with and without disabilities. For private commercial websites, the Department of Justice (DOJ), which enforces the ADA, has made it clear that it interprets the ADA as applicable to websites. In 2010, the DOJ issued an Advanced Notice of Proposed Rulemaking to specifically ensure all websites, public and private, are subject to ADA compliance. The DOJ’s proposed amendments to the ADA were initially expected in Spring of 2016 but have now been pushed back to 2018.
The Americans with Disabilities Act (ADA) and, if the government entities receive federal funding, the Rehabilitation Act of 1973 generally require that state and local governments provide qualified individuals with disabilities equal access to their programs, services, or activities unless doing so would fundamentally alter the nature of their programs, services, or activities or would impose an undue burden.2 One way to help meet these requirements is to ensure that government websites have accessible features for people with disabilities, using the simple steps described in this document. An agency with an inaccessible website may also meet its legal obligations by providing an alternative accessible way for citizens to use the programs or services, such as a staffed telephone information line. These alternatives, however, are unlikely to provide an equal degree of access in terms of hours of operation and the range of options and programs available.

I have previous experience with a company that sold a platform to help alleviate ADA compliance issues on websites. From my understanding litigation is being brought to a few industries with it beginning to trickle down into others. Government agencies, education, banking, and law firms were some of the early initiators that were paying more attention to ADA before any lawsuits approached their door.
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.