Yes, as of 2018 websites absolutely must be ADA compliant.  Its a new but very scary issue that has flared up over the last year and is set to cause a LOT of turmoil.  The Supreme court issued a judgement that opened up a loophole and opportunity-chasing lawyers are aggressively soliciting handicapped people and convincing them to sue unsuspecting website owners (pretty much every website owner on the planet, as not a single one of us were ADA compliant before now.)
Develop a plan for making your existing web content accessible. Describe your plan on an accessible webpage, and encourage input on how accessibility can be improved. Let visitors to your website know about the standards or guidelines that you are using to make your website accessible. When setting timeframes for accessibility modifications to your website, make more popular webpages a priority.
Technology is changing, and many website designers are using creative and innovative ways to present web-based materials. These changes may involve new and different access problems and solutions for people with disabilities. This Chapter discusses just a few of the most common ways in which websites can pose barriers to access for people with disabilities. By using the resources listed at the end of this Chapter, you can learn to identify and address other barriers.

Yes, in time non-ADA compliant websites will be penalized in the search engines. It's already on their radar. But for now they are still negatively affected, just in an indirect way. Live traffic behavior (user experience) is now factored into Google's algorithm and is a ranking signal. Negative user interactions absolutely have negative affects on rankings.
As organizations around the world scramble to bring their sites into compliance with the World Wide Web Consortium’s Web Content Accessibility Guidelines (WCAG), focus on other, preexisting accessibility regulations has also intensified. The United States’ Americans with Disabilities Act (ADA) is one of the most visible and complicated pieces of accessibility legislation. Let’s look at some of the ins and outs of what an ADA compliant website means today. Or, if you're interested in seeing the nitty-gritty details of your site's accessibility, request a free website audit report using the form on this page.
Do all websites have to be ADA compliant? Does yours? Though the legal definitions are somewhat unclear, it is clear that inaccessibility invites legal action and misgivings from customers. Web accessibility does not have to be complex, and it may not take much to test your site and make it accessible. Take web accessibility step by step and you can avoid stressful lawsuits, and invite all patrons to your website.
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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.
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."

While some might be tempted to do this if you understand anything about how Google indexes websites, someone who has thousands of pages on their site would lose rankings. The price of making your site fully accessible depends on how large your website is. Making the best decision for your specific situation is best done by consulting a professional.
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.