div {
  width: 100%;
  @media (min-width: 640px) {
    width: 50%;
  }
}

@layer demo {
  main {
    display: flex;
    flex-wrap: wrap;
  }
  div {
    border: medium solid black;
    height: 200px;
    box-sizing: border-box;
    display: inline-block;
  }
  body {
    display: grid;
    grid-template-columns: 250px 1fr;
    gap: 1em;
  }
  nav {
    background: teal;
  }
}