This jQuery plugin was developed for use in Accessible Web Design for users to increase and decrease their font size using on-page controls.
In order for this to work, all of your declared font sizes need to be sized relative to the body and cannot have absolute values, i.e. use ems or percentages instead of pixels.
Click the following links to adjust the size of the current page.
A APlease see the Github repository for instructions on using this plugin.