.foo {
  z-index: 0;
}

@media (min-width: 3em) and (min-width: 4em) {
  .foo {
    z-index: 1;
  }
}

@media (min-width: 2em) and (min-width: 5em) {
  .foo {
    z-index: 2;
  }
}

@media (min-width: 1em) and (min-width: 6em) {
  .foo {
    z-index: 3;
  }
}