&nbsp大小是不确定的 找到一个例子,可以参考这个 http://jsfiddle.net/nivas/CV5mQ/

至于其他2个 参考 https://www.w3.org/MarkUp/html3/specialchars.html

The character entities   and   denote an en space and an em space respectively, where an en space is half the point size and an em space is equal to the point size of the current font. For fixed pitch fonts, the user agent can treat the en space as being equivalent to a single space character, and the em space as being equuivalent to two space characters.