Bootstrap’s side project RFS is a unit resizing engine which was initially developed to resize font sizes (hence its abbreviation for Responsive Font Sizes). Nowadays RFS is capable of rescaling most CSS properties with unit values like margin, padding, border-radius, or even box-shadow.
The mechanism automatically calculates the appropriate values based on the dimensions of the browser viewport. It will be compiled into calc() functions with a mix of rem and viewport units to enable the responsive scaling behavior.
The mixins are included in Bootstrap and are available once you include Bootstrap’s scss. RFS can also be installed standalone if needed.
The rfs() mixin has shorthands for font-size, margin, margin-top, margin-right, margin-bottom, margin-left, padding, padding-top, padding-right, padding-bottom, and padding-left. See the example below for source Sass and compiled CSS.