Flexbox

body {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

footer {
  margin-top: auto;
}

// Optional
main {
  margin: 0 auto;
  // or: align-self: center
  max-width: 80ch;
}

Grid

body {
  min-height: 100vh;
  display: grid;
  grid-template-rows: auto 1fr auto;
}

// Optional
main {
  margin: 0 auto;
  max-width: 80ch;
}