h1 {
  text-align: center;
}

fieldset {
  width: 600px;
  margin-left: auto;
  margin-right: auto;  
}

label {
  float: left;
  width: 250px;
  clear: left;
  text-align: right;
  padding-right: 1em;
}

input {
  float: left;
}

button {
  display: block;
  clear: both;
  margin-left: auto;
  margin-right: auto;
}

#output {
  font-size: 150%;
  text-align: center;
  width: 400px;
  margin-left: auto;
  margin-right: auto;
}