html {
  font-family: sans-serif;
  min-width: 320px;
  max-width: 800px;
  margin: 0 auto;
  background-color: #333;
  color: #fff;
}
body{
  margin-bottom: 5rem;
}
header {
  text-align: center;
  margin-bottom: 2rem;
}

img {
  max-width: 100%;
}

.lettres {
  text-transform: uppercase;
  font-weight: bold;
}

input, button {
  font-size: 3rem;
  text-align: center;
  padding: .5rem;
}

input {
  width: 12rem;
  text-transform: uppercase;
}
.erreur {
  font-weight: bold;
  background-color: #880000;
  padding: .2em .5em;
}