Overlays are a broad term for technologies that aim to improve the accessibility of a website. They apply third-party source code (typically JavaScript) to make improvements to the front-end code of the website.
Website add-on products claiming to improve accessibility go back to the late 1990s with products like Readspeaker and Browsealoud. They added text-to-speech capabilities to the website(s) on which they were installed.
Then similar products came to market that added more tools to their software. These allow user-based control of things like font-sizes and colors to improve readability.
Some newer overlay products aim to fix any problems in the site's code that are preventing assistive technology from being used easily. They apply a script to the page which scans the code and automatically attempts to repair the problem.
Products like Userway, EqualWeb, AudioEye, User1st, MaxAccess, FACIL'iti, Purple Lens, and accessiBe are known as accessibility overlays. They are sometimes white labelled (sold under other names), so this is not a complete list of the type of products discussed on this page.
Overlay widgets are unnecessary and are poorly placed in the technology stack.
As stated above, some overlay products contain widgets which present a series of controls that modify the presentation of the page they're on. Depending on the product, those changes may do things like change the page contrast, enlarge the size of the page's text, or perform other changes to the page that are intended to improve the experience for users with disabilities.
To laypersons, these features may seem beneficial, but their practical value is largely overstated because the end users that these features claim to serve will already have the necessary features on their computer, either as a built-in feature or as an additional piece of software that the user needs to access not only the Web but all software.
On this latter point, it is a mistake to believe that the features provided by the overlay widget will be of much use by end users because if those features were necessary to use the website, they'd be needed for all websites that the user interacts with. Instead, the widget is —at best—redundant functionality with what the user already has.
While some automated repair is possible, customers should be discouraged from using an overlay as a long-term solution.
Some overlay products have capabilities aimed at providing accessibility repairs to the underlying page on which the overlay is added. These repairs are applied when the page loads in the user's browser.
While it is true that a non-trivial array of accessibility problems can be repaired in this manner, the nature, extent, and accuracy of such repair are limited by a number of important factors: