What are rem units? How to use them in CSS
rem units (short form of root-em) dictate the font size of an item relative to the size of the root item. By default, most browsers use a fontsize of 16 pixels. So, if the root element is 16px, an element with the value 1rem will also equal 16px.
rem units reference the size of the root element
Rem vs. Em Units
rem units are relative to the root element, em is relative to the parent element.