body {
  background: #333;
  color: white;
  font-family: Consolas, Monaco, monospace;
  font-size: 24px;
}

* { font: inherit }

#key { display: block; }

#key:invalid { color: red; }

#display {
  white-space: pre;
}

.centered {
  display: flex;
  justify-content: center;
  align-items: center;
  margin: 1em 0;
}