rem 自适应类

@media screen and (max-320px) {
    html {
        font-size: 32px
    }
}

@media screen and (min-321px) and (max-340px) {
    html {
        font-size: 34px
    }
}

@media screen and (min-341px) and (max-360px) {
    html {
        font-size: 36px
    }
}

@media screen and (min-361px) and (max-400px) {
    html {
        font-size: 40px
    }
}

@media screen and (min-401px) and (max-420px) {
    html {
        font-size: 42px
    }
}

@media screen and (min-421px) and (max-480px) {
    html {
        font-size: 46px
    }
}

@media screen and (min-481px) {
    html {
        font-size: 48px
    }
}