My plan is to make a control that allows the user to select the font size. It will be an accessibility feature.
Currently if you notice in Firefox, when viewing a page on this site, look at the View > Page Style menu and you will see you can select Normal Text, Medium Text, Large Text, which corresponds to these css files. (see the title attribute on the style link in source view)
Note that when viewing the source of the page, the link for styletext.css has rel='stylesheet' because it is the default one, while styletextmedium.css link has rel='alternate stylesheet'
Choosing the menu item only affects the current page, if you refresh or go to a different pages it goes back to using the default css. The planned control will likely use a cookie to preserve the user choice.
Joe