Currently, there is a safe harbor clause that allows your existing content to remain as it is, unless altered after January 18, 2018. However, the guidelines do pertain to any page that has been updated after that date. So if you want to avoid the legal costs of being found non-compliant with the ADA, it’s best to make the necessary changes to your website now.
...the full and equal enjoyment of the goods, services, facilities, or accommodations of any place of "public accommodation" by any person who owns, leases, or operates a place of public accommodation. Public accommodations include most places of lodging (such as inns and hotels), recreation, transportation, education, and dining, along with stores, care providers, and places of public displays.
State and local governments will often post documents on their websites using Portable Document Format (PDF). But PDF documents, or those in other image based formats, are often not accessible to blind people who use screen readers and people with low vision who use text enlargement programs or different color and font settings to read computer displays.
Since March 15, 2012, ADA compliance with the 2010 Standards will be required for new construction and alterations. In the period between September 15, 2010 and March 15, 2012, covered entities may choose between the 1991 Standards ADA Compliance (without the elevator exemption for Title II facilities), the Uniform Federal Accessibility Standards (Title II facilities only), and the 2010 Standards ADA Compliance.
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.”