css3 media

 @media screen and (max-width: 768px) {
        html {
          font-size: 30px !important;
        }
      }

      @media screen and (max-width: 420px) {
        html {
          font-size: 19px !important;
        }
      }

      @media screen and (max-width: 400px) {
        html {
          font-size: 18px !important;
        }
      }

      @media screen and (max-width: 320px) {
        html {
          font-size: 15px !important;
        }
      }