Synopsis: Both rem and em are units of size in CSS. Rem is the font-size of the root element. Em is the font-size of the local context.
The em unit is borrowed from the world of printed typography, and it is a unit that allows setting the font-size of an element relative to the nearest declared font-size in its component hierarchy.
The rem unit, short for root em, always references the font-size value of the root element, the <html> element. If the <html> element doesn’t have a specified font-size, the browser default of 16px is used.
Both of these units can be used for any sized attribute, such as margin and line-height. If so, rem maintains its consistent definition. Em, on the other hand, references the element's own font-size.
Developer's Take
I admit I used em a lot more in the code I've written, but after learning this, rem is more intuitive and reliable than em! I will be using rem from this time onward, unless I specifically want to take into account the sizing in the localized context.
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