Title III also has applications to existing facilities. One of the definitions of "discrimination" under Title III of the ADA is a "failure to remove" architectural barriers in existing facilities. See 42 U.S.C. § 12182(b)(2)(A)(iv). This means that even facilities that have not been modified or altered in any way after the ADA was passed still have obligations. The standard is whether "removing barriers" (typically defined as bringing a condition into compliance with the ADAAG) is "readily achievable", defined as "...easily accomplished without much difficulty or expense".
Your site would need to have a text only option, with all functionality accessible through a keyboard for visitors with mobility issues. Once you make these and other changes to meet ADA guidelines, your site would need to be tested by a website development company familiar with ADA compliance issues to be sure that visitors who use assistive technology such as screen readers are able to access your web content fully.
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).