.parent {
  width: 400px;
  height: 200px;
  display: table;
}

.child {
  width: 50%;
  display: table-cell;
  vertical-align: middle; /* center content vertically */
}

@media (max-width: 480px) {
  .parent {
    width: 100%;
    height: auto;
    display: block;
  }

  .child {
    width: 100%;
    display: block;
  }
}