如’cm’代表厘米。
我不是问什么’em’是或如何使用！
我在W3C找不到答案。
Wikipedia的文章已经在说“东西”：
The name of em is related to M. Originally the unit was derived from the width of the capital “M” in the given typeface.
我对第一句话的解释是“em”是字母“M”的发音/语音。
但阅读第二句话，似乎’e’在’em’代表一些关于字母’M’的宽度。
所以我还是迷路了，什么’em’真正代表！
解决方法
One em was traditionally defined as the width of the capital “M” in the current typeface and point size,as the “M” was commonly cast the full-width of the square “blocks”,or “em-quads” (also “mutton-quads”),which are used in printing presses. However,in modern typefaces,the character M is usually somewhat less than one em wide. Moreover,as the term has expanded to include a wider variety of languages and character sets,its meaning has evolved; this has allowed it to include those fonts,typefaces,and character sets which do not include a capital “M”,such as Chinese and the Arabic alphabet. Thus,em generally means the point size of the font in question,which is the same as the height of the Metal body a font was cast on.
特别是在CSS方面，“em”不一定指的是特定字体的大写字母M的宽度;它只是一个相对数量。
如果你问的是字词“em”的词源，维基百科本身只包含对Adobe Glossary的引用，其中没有更多的说法：
A common unit of measurement in typography. Em is traditionally defined as the width of the uppercase M in the current face and point size. It is more properly defined as simply the current point size. For example,in 12-point type,em is a distance of 12 points.
它没有明确提到任何权威，它是一个语音表示的首都M，但考虑到它的名字定义，我不会排除这种可能性。
