一. 概要

• em的部分实际应用与优缺点。

二.xxxx

The 'em' unit is equal to the computed value of the 'font-size' property of the element on which it is used. The exception is when 'em' occurs in the value of the 'font-size' property itself, in which case it refers to the font size of the parent element. ——css spec 2.1

The font size corresponds to the em square, a concept used in typography. ——css spec 2.1

CSS中，em是字体的名义点值或者英寸高度，实际上任何字体的物理尺寸依赖于the user-defined DPI setting,current element font-size,the particular font being used.——wiki

em用来代表当前字体设置的大小（point-size）——ps:1 point-size =1/72 in

A metal sort. The line height, c, is the precursor of the em.