Federal law isn't the only consideration for businesses. Additionally, each state interprets the law differently. Consider the case against Netflix in 2012. Lawsuits were brought in federal court in Massachusetts and California. Netflix was accused of violating the ADA by not offering "closed captioning" options for its Internet streamed movies. Illustrating the complexity of this issue, the courts reached completely opposite decisions. Massachusetts held that Netflix must comply with the ADA, while the California court found that Netflix did not fall under the ADA's definition of "public accommodation."
@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.
Federal law isn't the only consideration for businesses. Additionally, each state interprets the law differently. Consider the case against Netflix in 2012. Lawsuits were brought in federal court in Massachusetts and California. Netflix was accused of violating the ADA by not offering "closed captioning" options for its Internet streamed movies. Illustrating the complexity of this issue, the courts reached completely opposite decisions. Massachusetts held that Netflix must comply with the ADA, while the California court found that Netflix did not fall under the ADA's definition of "public accommodation."
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. 

Through the Americans with Disabilities Act (ADA), our nation committed itself to eliminating discrimination against people with disabilities. The U.S. Department of Justice’s (DOJ) Civil Rights Division is proud to play a critical role in enforcing the ADA, working towards a future in which all the doors are open to equality of opportunity, full participation, independent living, integration and economic self-sufficiency for persons with disabilities.
A one of a kind plug-in, WP ADA Compliance Check Basic does just that – checking WordPress websites for ADA compliance. There are two ways to use it. First, you can schedule a whole-site scan to find out any ADA compliance issues. Secondly, you can set it to run every time new content is published. When set this way, the plug-in will identify and report on any ADA compliance issues found in the new content as well as recommend possible solutions. The full version even corrects some of the uncovered issues automatically.
When’s the last time you thought about how accessible your website is to users who are blind or visually-impaired? Even though ADA website compliance is the law, many businesses have failed to update their websites to accommodate these users. Most recently, Winn-Dixie is in the news for failing to provide ADA compliant web design. A user who is blind was unable to use their site, filed a lawsuit, and a Miami judge ruled that Winn-Dixie violated the Americans with Disabilities Act. If your website is not currently accessible to the visually-impaired, it’s time to think about updating your site for compliance and user experience.

For people who have speech disabilities, this may include providing a qualified speech-to-speech transliterator (a person trained to recognize unclear speech and repeat it clearly) , especially if the person will be speaking at length, such as giving testimony in court, or just taking more time to communicate with someone who uses a communication board. In some situations, keeping paper and pencil on hand so the person can write out words that staff cannot understand or simply allowing more time to communicate with someone who uses a communication board or device may provide effective communication. Staff should always listen attentively and not be afraid or embarrassed to ask the person to repeat a word or phrase they do not understand.
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. 
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.
Until the ADA is updated to address the special case of website accessibility, or the Department of Justice releases its website accessibility regulations, complying with WCAG 2.0 Level AA is the best way to ensure that people with disabilities have equal access to your website. The overview below is a great starting point about meeting the WCAG 2.0 Level AA recommendations.

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.
In court, Netflix tried to argue that websites should not be part of ADA compliance regulations, as there is no physical structure / location. They also argued that websites should not be in scope of ADA as there is no public component (the original ADA compliance law specifically called out that ADA rules apply primarily to services, locations, and products that are supposed to be open to the public).

Attitudes toward digital accessibility have evolved over time. In 2010, the Justice Dept. said they would address the topic. They said the ADA covers web sites, including companies that don’t have brick and mortar presences. They said companies can comply with the law by offering an alternative means of access, so if a web site isn’t ADA compliant, they could offer 24/7 phone access as an example of alternative access.
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.
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