As I mentioned above, under each WCAG 2.1 principle is a list of guidelines, and under each guideline are compliance standards, with techniques and failure examples at each level. Some guidelines include only Level A items; others include items for multiple levels of conformance, building from A to AAA. At each stage, you can easily see what more you would need to do to reach Level AA or AAA. In this way, many websites include elements at multiple levels of accessibility.
As a result, most ADA suits are brought by a small number of private plaintiffs who view themselves as champions of the disabled. For the ADA to yield its promise of equal access for the disabled, it may indeed be necessary and desirable for committed individuals to bring serial litigation advancing the time when public accommodations will be compliant with the ADA."
Title III also has applications to existing facilities. One of the definitions of "discrimination" under Title III of the ADA is a "failure to remove" architectural barriers in existing facilities. See 42 U.S.C. § 12182(b)(2)(A)(iv). This means that even facilities that have not been modified or altered in any way after the ADA was passed still have obligations. The standard is whether "removing barriers" (typically defined as bringing a condition into compliance with the ADAAG) is "readily achievable", defined as "...easily accomplished without much difficulty or expense".
I should mention one caveat to all of this. Businesses that are required to comply but don't have the ability to bring their websites into compliance can provide an accessible alternative to provide the same information, goods, and services that they provide online, like a staffed phone line. The trick, however, is that this option has to provide at least equal access, including in terms of hours of operation. And, as we know, the internet is around 24/7, so good luck with that.
Certain specific conditions that are widely considered anti-social, or tend to result in illegal activity, such as kleptomania, pedophilia, exhibitionism, voyeurism, etc. are excluded under the definition of "disability" in order to prevent abuse of the statute's purpose. Additionally, gender identity or orientation is no longer considered a disorder and is also excluded under the definition of "disability".
Once you have been hired and started work, your employer cannot require that you take a medical examination or ask questions about your disability unless they are related to your job and necessary for the conduct of your employer's business. Your employer may conduct voluntary medical examinations that are part of an employee health program, and may provide medical information required by State workers' compensation laws to the agencies that administer such laws.