@media screen and (max-width: 900px) {
  .a {
    font-size: 12px;
  }
}
@media screen and (min-width: 900px) {
  .xyz {
    font-size: 12px;
  }
}
//@media screen and (min-width: 900px) and screen and (max-width: 900px) {
//  .xyz {
//    color: red;
//  }
//}