.foo {
  z-index: 0;
}

@media screen and (min-width: 2em) {
  .foo {
    z-index: 2;
  }
}

@media all, (min-width: 3em) {
  .foo {
    z-index: 3;
  }
}

@media (min-width: 1em) and (min-height: 1em) {
  .foo {
    z-index: 1;
  }
}