The rem CSS unit allows you to base your font sizes off of the user’s configured font size, but not be affected by parent elements like ems are. Older browsers don’t support rems though, so it’s good to provide a fallback in px for them by defining the property with a px value, then a rem value. Old browsers take the px value, then see they don’t know how to handle the rem value and ignore it. New browsers take the px value, then override it with the rem value.
There are a number of SCSS mixins out there for making rem fallbacks automatically by passing in a property and some values and having it automatically output both versions of the property. I started with the css-tricks one. None of the ones I found, though, worked exactly as I wanted. I wanted to be able to work with any property values. I wanted:
unitless number values (other than 0) to be treated as px values
px or rem values to be converted to the other unit
values with other units or non-numbers to be output as is (auto, none, url, etc)