html,
body {
  min-height: 100%;
  margin: 0;
  background: transparent;
}

body {
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: -apple-system, BlinkMacSystemFont, sans-serif;
}

#challenge {
  width: min(100%, 300px);
  min-height: 70px;
}