@counter-style my-alpha {
  system: fixed;
  symbols: Ⓐ Ⓑ Ⓒ;
  suffix: " ";
}

@media (max-width: 400px) {

  @supports (display: flex) {
    @counter-style my-alpha {
      system: fixed;
      symbols: a b c;
      suffix: " ";
    }
  }

  @counter-style my-alpha {
    system: fixed;
    symbols: Ⓐ Ⓑ Ⓒ;
    suffix: " ";
  }
}