A common recommendation for designing Web sites for people with cognitive disabilities is to use large fonts and/or to enable the enlargement of them. See WebAIM: Cognitive Web Accessibility Checklist and Inclusive New Media Design: Top Tips.
Though Web browsers enable resizing of a Web site’s default font, it is widely accepted by the community of Web accessibility professionals that most people do not know the feature exists. This is the primary rationale for the placement of font-size switchers on Web sites.
For several years, I have incorporated a font-size switcher into the Web sites I have designed. An example is in use on the Web site of the Massachusetts Disability Information Locator (MADIL). It is located on the right side of the site’s pages, just under the site-navigation tabs. It has five font-size selectors, including one that changes the page background color to black.
I will soon experiment with adding this font-size switcher to the “Clear Helper” home page. In the interest of reducing the number of distractions, also a Web-accessibility recommendation for people with cognitive disabilities, I may limit the number of its selectors to three. One will reduce the font to the same size as the default font of the visitor’s Web browser; one will make the font even larger than the site’s standard (large) font; and one will set/restore the font size to the site’s standard. The way this works should become more clear once the font-size switcher is in action.