http://www.w3cplus.com/css3/define-font-size-with-css3-rem
https://mp.weixin.qq.com/s/DpLXJhfCHsgrbgbuk25Ekw
https://github.com/flashlizi/cssrem
http://www.w3cplus.com/css/when-to-use-em-vs-rem.html
http://pxtoem.com/
http://www.cnblogs.com/xcsn/p/4922562.html