One of the main advantages of using relative units is the cascade effect generated when the user modifies the font size of the base element (from the browser) and all font sizes increase/decrease proportionally.
These days, almost every browser has the default settings of this base element <html>
at 16 px. However, this value can be modified on the user side if the user wants to increase the browser font size making it easier to read.
Before talking about the most commonly used measuring units, there are two units we would emphasize because their popularity has grown impressively, and they are: vw (viewport width) and vh (viewport height).
These viewport units still do not have much acceptance for most used browsers, but I suggest you check either http://www.w3.org/TR/css3-values/ or http://caniuse.com/viewport-units to stay tuned because these units make the scale of the font size proportionally easier depending on the size of the...