The World Wide Web Consortium (W3C) established the main international standards and accessibility for the World Wide Web. The WCAG is created by the W3C to provide a standard for web content accessibility that can be shared around the world. The WCAG is meant to accompany organizations as a sort of blueprint on how to make their websites ADA compliant.

The words in the tag should be more than a description. They should provide a text equivalent of the image. In other words, the tag should include the same meaningful information that other users obtain by looking at the image. In the example of the mayor’s picture, adding an “alt” tag with the words “Photograph of Mayor Jane Smith” provides a meaningful description.
In 2001, for men of all working ages and women under 40, Current Population Survey data showed a sharp drop in the employment of disabled workers, leading at least two economists to attribute the cause to the Act.[52] By contrast, a study in 2003 found that while the Act may have led to short term reactions by employers, in the long term, there were either positive or neutral consequences for wages and employment.[53] In 2005 the rate of employment among disabled people increased to 45% of the population of disabled people.[54]
This is the reason why I think web design firms have to position themselves as solution providers and not just web guys/gals. A client doesn't need a website, they need sales. The website is just part of that solution but it's not the solution and unlike web design, service like email marketing automation, SEO, and PPC requires an experienced professional.
Google and the government are already making asks around their expectations of accessibility as a cornerstone of website development. Google already rewards sites that check many accessibility boxes with higher rankings and the government has signaled that web accessibility standards will be required for government contractors. By building a more accessible website now, you may significantly impact your SEO and ranking potential, not to mention open the doors to possible work as a government contractor.
Access Now v. Southwest Airlines was a case where the District Court decided that the website of Southwest Airlines was not in violation of the Americans with Disabilities Act, because the ADA is concerned with things with a physical existence and thus cannot be applied to cyberspace. Judge Patricia A. Seitz found that the "virtual ticket counter" of the website was a virtual construct, and hence not a "public place of accommodation." As such, "To expand the ADA to cover 'virtual' spaces would be to create new rights without well-defined standards."[76]
The debate over the Americans with Disabilities Act led some religious groups to take opposite positions.[32] The Association of Christian Schools International, opposed the ADA in its original form.[33] primarily because the ADA labeled religious institutions "public accommodations", and thus would have required churches to make costly structural changes to ensure access for all.[34] The cost argument advanced by ACSI and others prevailed in keeping religious institutions from being labeled as "public accommodations".[24]
Level AA is a little more significant, and makes sites accessible to people with a wider range of disabilities, including the most common barriers to use. It won't impact the look and feel of the site as much as Level AAA compliance, though it does include guidance on color contrast and error identification. Most businesses should be aiming for Level AA conformity, and it appears to reflect the level of accessibility the DOJ expects. 

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.  
The ADA defines a covered disability as a physical or mental impairment that substantially limits one or more major life activities, a history of having such an impairment, or being regarded as having such an impairment. The Equal Employment Opportunity Commission (EEOC) was charged with interpreting the 1990 law with regard to discrimination in employment. The EEOC developed regulations limiting an individual's impairment to one that "severely or significantly restricts" a major life activity. The ADAAA directed the EEOC to amend its regulations and replace "severely or significantly" with "substantially limits", a more lenient standard.[42]