div {
  aspect-ratio: 1 / 1; /* same as "1" or "42 / 42" */
  
}

main {
  max-inline-size: 40ch;
  margin-inline: auto;
  min-block-size: 100dvh;
  background: white;
  padding: 1rem;
}
body {
  background: #eee;
  margin: 0;
}
* {
  box-sizing: border-box;
}
div {
  display: grid;
  place-content: center;
  background: red;
  color: white;
}