The difference between EM and rem in CSS

The maximum unit length in CSS is px, EM, REM, the three differences are:
1.PX is a fixed pixel, and once set it cannot be changed to fit the page size.
2.em and REM are more flexible relative to PX, they are relative length units, meaning that the length is not dead, and more suitable for responsive layouts.

The difference between EM and REM is summed up in a nutshell: Em relative to the parent element, Rem relative to the root element.
The R in rem means root root.

Em:
1. Child element font size EM is relative to parent element font size
2. The width/height/padding/margin of the element with the EM word is relative to the font-size of the element

Rem:

You can set the size in the head in HTML.

The difference between EM and rem in CSS