"Mobile" mobile-side font units font-size select px or REM

For a small number of mobile devices only need to fit, and the resolution of the page has little impact, the use of PX can
For devices that need to be adapted to a variety of mobile devices, use REM, such as a device that only needs to be adapted to different resolutions, such as iphone and ipad.

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}}

"Mobile" mobile-side font units font-size select px or REM