CSS: em vs. rem

1. both

Font size units.

Flexible, scalable units which are translated by the browser into pixel values,

Both rem and em are relative units, px is not.

 

2. em

   : em inherits and scales relative to the font size of the element on which they are used.

   : Assumes the parent is 1em(100%). Children inherit size by scaling in relation to the parent size.
   : The name em was originally a reference to the width of the capital M in the typeface and size being used, which was often the same as the point size.

 

3. rem

   : rem is only relative to the root(HTML). Root em