移动端font-size适配

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