Government Code §11546.7 – The requirement that state agency heads certify, every two years, that their agency’s website meets the Web Content Accessibility Guidelines, Version 2.0 or a subsequent version, at Level AA or higher, and the requirements of Sections 11135 and 7405 of the Government Code. Created by AB 434 (Baker, Chapter 780, Statutes of 2017), and sometimes referred to as AB 434.
While legal considerations might be your biggest worry, making your site more accessible is simply good customer service. More than 39 million Americans are blind and another 246 million have "low vision," Another one million are deaf in the U.S. Add to that people with mobility issues that prevent them from using their hands and that's a huge portion of the country's buying power.
The gray area is a matter of scale and purpose. If you have a small business which serves as specific market it's a good idea to make the site as user and disability friendly as possible. If, however, your market serves segments with a likeliness to have various handicaps or disabilities you should address that with an ADA Compliance review of the site. Also consider the scale and purpose of an organization. Publicly traded large businesses need to concern themselves more than a single location family owned retailer and if someone sues your flower shop because the text was too small or didn't have "image alt tags" causing them distress... That's pretty frivolous and would likely not stand.
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.
Title III of the ADA requires that every owner, lessor, or operator of a “place of public accommodation” provide equal access to users who meet ADA standards for disability. With roughly 1.66 billion people around the world making online purchases in 2017, one might reasonably presume that this concept extends to websites, but from a legal standpoint, there is a surprising amount of grey area.
Most recently, however, pizza chain Domino's has been brought under suit for their website not being accessible for specialty ordering. The U.S. Supreme Court refused to review the case, instead upholding the decision of the 9th U.S. Circuit Court of Appeals who said the “alleged inaccessibility of Domino’s website and app impedes access to the goods and services of its physical pizza franchises—which are places of public accommodation.”