@media (max-width: 800px) {
  main {
    width: 100%;
    padding: 2rem;
  }
}
@media (max-width: 720px) {
  .tip-calculator {
    flex-direction: column;
  }
  #sub-num {
    text-align: center;
  }
  button {
    margin-top: 3rem;
  }
}
@media (max-width: 420px) {
  main {
    padding: 1rem 0 0 0;
  }
  .tip-calculator {
    border-bottom-right-radius: 0;
    border-bottom-left-radius: 0;
  }
  #errormsg {
    font-size: 12px;
  }
}