Reading/Web/Experiments/MobileFrontend Font Size Changer
The font size changer was an experiment in the year 2015 on MobileFrontend that adds a font size selector on the settings page that you can open from the menu.
This allows the user to select a bigger or smaller font to improve the readability of the site according to user's preferences.
Status edit
The feature first was on the Beta mode of MobileFrontend and was promoted to the stable mode of MobileFrontend.