手机自适应 rem

@media only screen and (min-width: 320px){
  html {
    font-size: 62.5% !important;
  }
}
@media only screen and (min-width: 640px){
  html {
    font-size: 125% !important;
  }
}
@media only screen and (min-width: 750px){
  html {
    font-size: 150% !important;
  }
}
@media only screen and (min-width: 1242px){
  html {
    font-size: 187.5% !important;
  }
}