Up-to-date standards are offered for checking compliance, and it provides a cost-effective way to scan a site for compliance. There's services out there where it'd cost more than $150 (the current cost of a 1 site 1 year license) for the size of the sites I'm working with. I'm very happy to see this offering in the first place, and it's even nicer to see how thorough it looks to be. Of course, it's not perfect so I'd love to see more of the WP.org listing fleshed out (the developer author/contributor mention is missing, etc.), I'd love to see the free version on GitHub (or similar for community development assistance [I have a few things I'd help with] which is very nice to see in a WP.org listing to show there's support / ongoing development for the plugin), and of course I'd love to see the server-side checker come to feature-parity with WAVE (currently WAVE is showing a few things the server-side checker doesn't have [color contrast, etc.]) Keep up the great work!

A person’s method(s) of communication are also key. For example, sign language interpreters are effective only for people who use sign language. Other methods of communication, such as those described above, are needed for people who may have lost their hearing later in life and do not use sign language. Similarly, Braille is effective only for people who read Braille. Other methods are needed for people with vision disabilities who do not read Braille, such as providing accessible electronic text documents, forms, etc., that can be accessed by the person’s screen reader program.
Higher search engine rankings is another potential benefit of voluntary compliance with the ADA. During your migration, an ADA specialist will audit it to detect and address usability challenges. These difficulties include the inability for search engine spiders to index your site easily. When you address such issues, your site will rank highly in search engines.
The landscape of disabled access litigation related to online services has significantly changed and expanded over the past decade. Initially, the internet was an area of little concern as courts uniformly held that the ADA applied to "brick and mortar" facilities, not to cyberspace. This has changed and online accessibility is presently, and will continue to be, an area of significant investigation and litigation.

Imagine struggling to do something as simple as navigating and reading a website, when you’re suffering from Glaucoma, Cataracts, Macular degeneration, Retinal disorders, Refractive errors, Optic nerve disorders, and other eye issues. Reading the text in order to gain knowledge and information, would be tiring at the least. Your website doesn’t need to be like that, in fact, your website can be a pleasant experience where the visually impaired can easily zoom in and out on every page or, the text to speech feature will convert to the audible voice that you are listening to right now.
Accessible design benefits all users and creates market opportunity, Reha said. She continued: “Access to information online is everywhere, so it’s easy to take for granted that it’s available to everyone, but it’s not. Such access is incredibly important for people with disabilities because it lets them become independent and improves their quality of life.”
This is particularly important when working for a government agency or government contractor, as these organizations must follow web accessibility guidelines under Section 508 of the Workforce Rehabilitation Act of 1973. Although ADA and Section 508 compliance are different, the published checklist for Section 508 compliance offers insight into ways to make websites accessible for people with disabilities, and thereby work toward 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.  
The Department has assembled an official online version of the 2010 Standards to bring together the information in one easy-to-access location.  It provides the scoping and technical requirements for new construction and alterations resulting from the adoption of revised 2010 Standards in the final rules for Title II (28 CFR part 35) and Title III (28 CFR part 36).
@Rick, I didn't specifically look at doctype, but Steve Faulkner did in his original article (see the link at the top of this post). He found that 101 of the 130 pages containing role="main" (78%) used the HTML5 doctype, which of course leaves 22% that didn't. I do recommend using ARIA even if using an earlier doctype. Validation is important, but if validation and accessibility conflict, I think most would agree accessibility should trump validation. Assistive technologies that support ARIA landmarks do so regardless of doctype, and otherwise the presence of these role attributes has no ill effects in any user agent.
The ADA guidelines provide the foundation that organizations need to achieve digital accessibility best approaches, however they are not exhaustive. They do not provide direction for all of the accessibility challenges that people with disabilities face. They also fail to provide detailed technical instructions. With these limitations in mind, there is a ray of hope.
The pressure is mounting to implement technology that more directly connects with residents, but balancing the shift to enable all citizens – including the approximately 19 percent with disabilities – can seem insurmountable. Beyond ensuring your organization is doing right your community, being in compliance with the Americans with Disabilities Act (ADA) or Section 508 standards is daunting – especially with the threat of a fine or lawsuit for non-compliance.

When you insert an image into a post or page, consider providing a rich description for the caption that will improve the reading experience for everyone, but especially folks who can’t see the image. Be creative. Instead of “My son on his swing,” try “My son is playing on his favorite swing. His face is filled with pure joy on a beautiful Spring day. Perfection.” The goal here is to convey the feeling of the image.