em,rem

em rem 相对单位:  也可用于设置padding line-height等
em相对当前容器的默认字体设置
比如,所有浏览器默认字体都是16px,body{ font-size:62.5%}
以后即1em=10px; 1.2em=12px
在线转换tool: http://pxtoem.com/

em(css2.0) rem(css3.0)

rem相对于页根元素的字体大小，即 html 元素的字体大小
比如:html{font-size:16px;} 10rem=160px;
配合 使用media query 设置html的font-size;

html{font-size:10px}
@media screen and (min-width:321px) and (max-width:375px){html{font-size:11px}}
@media screen and (min-width:376px) and (max-width:414px){html{font-size:12px}}
@media screen and (min-width:415px) and (max-width:639px){html{font-size:15px}}
@media screen and (min-width:640px) and (max-width:719px){html{font-size:20px}}
@media screen and (min-width:720px) and (max-width:749px){html{font-size:22.5px}}
@media screen and (min-width:750px) and (max-width:799px){html{font-size:23.5px}}
@media screen and (min-width:800px){html{font-size:25px}}