EM vs REM

EM is relative to the parent element of the Font-size,rem is relative to the root element of the font-size.

The supplement of REM:

① for browsers that do not support it, the workaround is simple enough to write an absolute unit statement. These browsers ignore the font size set by REM;

②ie9/ie10 does not support REM when used for pseudo-elements or when using a font shorthand declaration;

③ios safari5.0-5.1, although REM is supported, does not support REM when using media queries.

EM vs REM