EM 相对于其父级字体大小

EMs are relative to their parent's font size

REM base font-size

REMs are relative to a base font-size

当中间容器更改字体大小时，这一点很重要。具有EM的子元素将受到影响，使用REM的子元素将不受影响。

This is important when intermediate containers change font sizes. Child elements with EMs will be affected, those using REMs will not.