h1,
form,

#btn 
{
  width: 395px;
  margin: 2rem auto;
  font: 16px sans-serif;
}
#qrcode
{
  width: 260px;
  margin: 2rem auto;
  font: 16px sans-serif;
}


h1 {
  font-size: 64px;
  border-bottom: 1px solid black;
}

h1,
label,
button {
  display: flex;
  width: 100%;
  margin-bottom: 1rem;
  justify-content: center;
}

label > * { flex: 1; }
label > span {
  max-width: 8rem;
  margin-right: 1rem;
}